Test your ears.

gehoertest.spin

CON
  _clkmode = xtal1 + pll16x     ' 16x the input freq
  _xinfreq = 5_000_000          ' 5 MHz crystal                                  
  speaker = 11                  ' where the speaker is connected
OBJ
  pst : "Parallax Serial Terminal"                      ' this is for debugging
VAR
  WORD freq                     ' actual frequency
  WORD d                        ' difference between the frequencies                                                
  LONG fx                       ' frequency value for the frqa register
PUB main
  dira[speaker]~~               ' speaker is an output
  d := 10                       ' initial value
  pst.start(115_200)            ' initialize serial connection with 115_200 baud
  ctra[30..26] := %00100        ' use as NCO
  ctra[5..0] := speaker         ' output for the NCO                                 
  repeat freq from 10 to 20_000 step d                  ' frequencies go from 10 Hz to 20 kHz, delta will be adjusted on the fly                  
    pst.dec(freq)               ' print the frequency on the terminal
    pst.str(String(pst#NL))     ' ...and a newline
    case freq                   ' adjusting the freq delta
      10..99         : d := 10
      100..299       : d := 20
      300..499       : d := 50
      500..999       : d := 100
      1_000..4_999   : d := 500
      5_000..20_000  : d := 1000
    fx := ($8000_0000 / (clkfreq / freq)) << 1          ' converting freq for the NCO
    frqa := fx                  ' set the freq                                                                                          
    waitcnt(clkfreq+cnt)        ' wait a second