From http://stackoverflow.com/questions/11355353/how-can-i-convert-qbasic-play-commands-to-something-more-contemporary

play2wav.c

// file: play2wav.c
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>

#ifndef M_PI
#define M_PI 3.14159265358
#endif

double Note2Freq(int Note) // Note=1 = C1 (32.7032 Hz), Note=84 = B7 (3951.07 Hz)
{
  double f = 0;
  if (Note > 0)
    f = 440 * exp(log(2) * (Note - 46) / 12);
  return f;
}

int Name2SemitonesFromC(char c)
{
  static const int semitonesFromC[7] = { 9, 11, 0, 2, 4, 5, 7 }; // A,B,C,D,E,F,G
  if (c < 'A' && c > 'G') return -1;
  return semitonesFromC[c - 'A'];
}

typedef struct tPlayer
{
  enum
  {
    StateParsing,
    StateGenerating,
  } State;

  int Tempo;
  int Duration;
  int Octave;
  enum
  {
    ModeNormal,
    ModeLegato,
    ModeStaccato,
  } Mode;

  int Note;
  double NoteDuration;
  double NoteTime;
  unsigned SampleRate;
} tPlayer;

void PlayerInit(tPlayer* pPlayer, unsigned SampleRate)
{
  pPlayer->State = StateParsing;
  pPlayer->Tempo = 120; // [32,255] quarter notes per minute
  pPlayer->Duration = 4; // [1,64]
  pPlayer->Octave = 4; // [0,6]
  pPlayer->Mode = ModeNormal;
  pPlayer->Note = 0;
  pPlayer->SampleRate = SampleRate;
}

int PlayerGetSample(tPlayer* pPlayer, const char** ppMusicString, short* pSample)
{
  int number;
  int note = 0;
  int duration = 0;
  int dotCnt = 0;
  double sample;
  double freq;

  *pSample = 0;

  while (pPlayer->State == StateParsing)
  {
    char c = **ppMusicString;

    if (c == '\0') return 0;

    ++*ppMusicString;

    if (isspace(c)) continue;

    c = toupper(c);

    switch (c)
    {
    case 'O':
      c = **ppMusicString;
      if (c < '0' || c > '6') return 0;
      pPlayer->Octave = c - '0';
      ++*ppMusicString;
      break;

    case '<':
      if (pPlayer->Octave > 0) pPlayer->Octave--;
      break;

    case '>':
      if (pPlayer->Octave < 6) pPlayer->Octave++;
      break;

    case 'M':
      c = toupper(**ppMusicString);
      switch (c)
      {
      case 'L':
        pPlayer->Mode = ModeLegato;
        break;
      case 'N':
        pPlayer->Mode = ModeNormal;
        break;
      case 'S':
        pPlayer->Mode = ModeStaccato;
        break;
      case 'B':
      case 'F':
        // skip MB and MF
        break;
      default:
        return 0;
      }
      ++*ppMusicString;
      break; // ML/MN/MS, MB/MF

    case 'L':
    case 'T':
      number = 0;
      for (;;)
      {
        char c2 = **ppMusicString;
        if (isdigit(c2))
        {
          number = number * 10 + c2 - '0';
          ++*ppMusicString;
        }
        else break;
      }
      switch (c)
      {
      case 'L':
        if (number < 1 || number > 64) return 0;
        pPlayer->Duration = number;
        break;
      case 'T':
        if (number < 32 || number > 255) return 0;
        pPlayer->Tempo = number;
        break;
      }
      break; // Ln/Tn

    case 'A': case 'B': case 'C': case 'D':
    case 'E': case 'F': case 'G':
    case 'N':
    case 'P':
      switch (c)
      {
      case 'A': case 'B': case 'C': case 'D':
      case 'E': case 'F': case 'G':
        note = 1 + pPlayer->Octave * 12 + Name2SemitonesFromC(c);
        break; // A...G
      case 'P':
        note = 0;
        break; // P
      case 'N':
        number = 0;
        for (;;)
        {
          char c2 = **ppMusicString;
          if (isdigit(c2))
          {
            number = number * 10 + c2 - '0';
            ++*ppMusicString;
          }
          else break;
        }
        if (number < 0 || number > 84) return 0;
        note = number;
        break; // N
      } // got note #

      if (c >= 'A' && c <= 'G')
      {
        char c2 = **ppMusicString;
        if (c2 == '+' || c2 == '#')
        {
          if (note < 84) note++;
          ++*ppMusicString;
        }
        else if (c2 == '-')
        {
          if (note > 1) note--;
          ++*ppMusicString;
        }
      } // applied sharps and flats

      duration = pPlayer->Duration;

      if (c != 'N')
      {
        number = 0;
        for (;;)
        {
          char c2 = **ppMusicString;
          if (isdigit(c2))
          {
            number = number * 10 + c2 - '0';
            ++*ppMusicString;
          }
          else break;
        }
        if (number < 0 || number > 64) return 0;
        if (number > 0) duration = number;
      } // got note duration

      while (**ppMusicString == '.')
      {
        dotCnt++;
        ++*ppMusicString;
      } // got dots

      pPlayer->Note = note;
      pPlayer->NoteDuration = 1.0 / duration;
      while (dotCnt--)
      {
        duration *= 2;
        pPlayer->NoteDuration += 1.0 / duration;
      }
      pPlayer->NoteDuration *= 60 * 4. / pPlayer->Tempo; // in seconds now
      pPlayer->NoteTime = 0;

      pPlayer->State = StateGenerating;
      break; // A...G/N/P

    default:
      return 0;
    } // switch (c)
  }

  // pPlayer->State == StateGenerating
  // Calculate the next sample for the current note

  sample = 0;

  // QuickBasic Play() frequencies appear to be 1 octave higher than
  // on the piano.
  freq = Note2Freq(pPlayer->Note) * 2;

  if (freq > 0)
  {
    double f = freq;

    while (f < pPlayer->SampleRate / 2 && f < 8000) // Cap max frequency at 8 KHz
    {
      sample += exp(-0.125 * f / freq) * sin(2 * M_PI * f * pPlayer->NoteTime);
      f += 2 * freq; // Use only odd harmonics
    }

    sample *= 15000;
    sample *= exp(-pPlayer->NoteTime / 0.5); // Slow decay
  }

  if ((pPlayer->Mode == ModeNormal && pPlayer->NoteTime >= pPlayer->NoteDuration * 7 / 8) ||
      (pPlayer->Mode == ModeStaccato && pPlayer->NoteTime >= pPlayer->NoteDuration * 3 / 4))
    sample = 0;

  if (sample > 32767) sample = 32767;
  if (sample < -32767) sample = -32767;

  *pSample = (short)sample;

  pPlayer->NoteTime += 1.0 / pPlayer->SampleRate;

  if (pPlayer->NoteTime >= pPlayer->NoteDuration)
    pPlayer->State = StateParsing;

  return 1;
}

int PlayToFile(const char* pFileInName, const char* pFileOutName, unsigned SampleRate)
{
  int err = EXIT_FAILURE;
  FILE *fileIn = NULL, *fileOut = NULL;
  tPlayer player;
  short sample;
  char* pMusicString = NULL;
  const char* p;
  size_t sz = 1, len = 0;
  char c;
  unsigned char uc;
  unsigned long sampleCnt = 0, us;

  if ((fileIn = fopen(pFileInName, "rb")) == NULL)
  {
    fprintf(stderr, "can't open file \"%s\"\n", pFileInName);
    goto End;
  }

  if ((fileOut = fopen(pFileOutName, "wb")) == NULL)
  {
    fprintf(stderr, "can't create file \"%s\"\n", pFileOutName);
    goto End;
  }

  if ((pMusicString = malloc(sz)) == NULL)
  {
NoMemory:
    fprintf(stderr, "can't allocate memory\n");
    goto End;
  }

  // Load the input file into pMusicString[]

  while (fread(&c, 1, 1, fileIn))
  {
    pMusicString[len++] = c;

    if (len == sz)
    {
      char* p;

      sz *= 2;
      if (sz < len)
        goto NoMemory;

      p = realloc(pMusicString, sz);
      if (p == NULL)
        goto NoMemory;

      pMusicString = p;
    }
  }

  pMusicString[len] = '\0'; // Make pMusicString[] an ASCIIZ string

  // First, a dry run to simply count samples (needed for the WAV header)

  PlayerInit(&player, SampleRate);
  p = pMusicString;
  while (PlayerGetSample(&player, &p, &sample))
    sampleCnt++;

  if (p != pMusicString + len)
  {
    fprintf(stderr,
            "Parsing error near byte %u: \"%c%c%c\"\n",
            (unsigned)(p - pMusicString),
            (p > pMusicString) ? p[-1] : ' ',
            p[0],
            (p - pMusicString + 1 < len) ? p[1] : ' ');
    goto End;
  }

  // Write the output file

  // ChunkID
  fwrite("RIFF", 1, 4, fileOut);

  // ChunkSize
  us = 36 + 2 * sampleCnt;
  uc = us % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);

  // Format + Subchunk1ID
  fwrite("WAVEfmt ", 1, 8, fileOut);

  // Subchunk1Size
  uc = 16;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);
  fwrite(&uc, 1, 1, fileOut);
  fwrite(&uc, 1, 1, fileOut);

  // AudioFormat
  uc = 1;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);

  // NumChannels
  uc = 1;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);

  // SampleRate
  uc = SampleRate % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = SampleRate / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);
  fwrite(&uc, 1, 1, fileOut);

  // ByteRate
  us = (unsigned long)SampleRate * 2;
  uc = us % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);

  // BlockAlign
  uc = 2;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);

  // BitsPerSample
  uc = 16;
  fwrite(&uc, 1, 1, fileOut);
  uc = 0;
  fwrite(&uc, 1, 1, fileOut);

  // Subchunk2ID
  fwrite("data", 1, 4, fileOut);

  // Subchunk2Size
  us = sampleCnt * 2;
  uc = us % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);
  uc = us / 256 / 256 / 256 % 256;
  fwrite(&uc, 1, 1, fileOut);

  // Data
  PlayerInit(&player, SampleRate);
  p = pMusicString;
  while (PlayerGetSample(&player, &p, &sample))
  {
    uc = (unsigned)sample % 256;
    fwrite(&uc, 1, 1, fileOut);
    uc = (unsigned)sample / 256 % 256;
    fwrite(&uc, 1, 1, fileOut);
  }

  err = EXIT_SUCCESS;

End:

  if (pMusicString != NULL) free(pMusicString);
  if (fileOut != NULL) fclose(fileOut);
  if (fileIn != NULL) fclose(fileIn);

  return err;
}

int main(int argc, char** argv)
{
  if (argc == 3)
//    return PlayToFile(argv[1], argv[2], 44100); // Use this for 44100 sample rate
    return PlayToFile(argv[1], argv[2], 16000);

  printf("Usage:\n  play2wav <Input-QBASIC-Play-String-file> <Output-Wav-file>\n");
  return EXIT_FAILURE;
}

Some code reading notes:

1. Player->State is a way to combine iteration (do we have more music to parse) with the data, and it could be refactored outside the while loop on Line #339.

2. The file reading part is a little bit more complicated than required; I'd have used fseek, ftell, malloc to read the contents into a buffer; Line #311; Read all bytes into a char array, basically.

3. The command parsing/dispatch code in Line #87 is explained as follows:

O? - Set Octave to ?
< - Reduce Octave
> - Increase Octave

L? - Length/Duration to Play Note as Specified by ? (1, 1/4 etc.),
M? - Set Mode based on ?, one of:
L - Legato (Full Length of Note set by L)
N - Normal (7/8th of the Length of Note set by L)
S - Staccato (3/4th of the Length of Note set by L)
B - Play music in Background (Another Thread) - Hence Ignored
F - Play music in foreground (Same Thread) - Hence Ignored

. - Note Length setter. The more dots, the Length of the note goes up with each element in the series 1/2 + 1/4 + 1/8

T? - Set Tempo to Tempo specified by ?

P? - Pause for the duration Specified by ? (no music played)

A?
B?
C?
D?
E?
F?
G? - Actual Note in currently set Octave ? = + # (Sharp) - (Flat), a half note higher or lower
N? - Set Note to ? between [0, 84)

4. Now, for each note, you map it to a number based on Octave, Sharp, then to the frequency based on an exponential scale on Line #12.

5. You generate the waveform at every point in this Line #254; You add a exponential dampening to it in Line #259; Finally, you take into account the Staccato/Legato and truncate as required in Line #262.

6. Then you clip it in Line #266.

7. You write this as a 16 bit value in Line #444; The WAV File header is itself written in Line #356.

8. You keep repeating this thanks to the while loop. There are two passes of this data done; One to simply get the time/length in Line #339 and the second to export out the sample itself into the WAV file in Line #441.