arduino.c

#include "FastLED.h"
#define NUM_LEDS 120
#define DATA_PIN 6

CRGB leds[NUM_LEDS];

void setup() { 
  FastLED.addLeds<NEOPIXEL, DATA_PIN>(leds, NUM_LEDS); 
  for(int i = 0; i < NUM_LEDS; i++) {
    leds[i].red = 5;
    leds[i].green = 0;
    leds[i].blue = 0;
  }
  pinMode(13, OUTPUT);
  Serial.begin(115200);
}



void loop() {
  if (Serial.available() > 0) {
    for(int i = 0; i < NUM_LEDS; i++) {

      int data =  -1;

      while(data < 0) {
        data = Serial.read();
      }
      // read the incoming byte:

      byte hue = (byte)(data * .375);
      
      
      leds[i] = CHSV(96 - hue, 255 - hue, max(data, 5));
    }
  }
  FastLED.show();
}

server.cs

using System;
using System.Collections.Generic;
using System.IO.Ports;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using NAudio.Wave.SampleProviders;
using NAudio.Wave;
using System.IO.Pipes;

namespace SerialThing
{
    class Program
    {
        const byte NUM_LEDS = 120;
        const int MAX_FREQ = 360;

        const int factor = (int)((float)MAX_FREQ / NUM_LEDS);

        static void Main(string[] args)
        {
            SampleAggregator sampleAggregator = new SampleAggregator(2048);

            byte[] data = new byte[NUM_LEDS];
            sampleAggregator.FftCalculated += (s, e) => {
                var newData = new byte[NUM_LEDS];
                for(var i = 0; i < NUM_LEDS; i++)
                {
                    newData[i] = (byte)(data[i] * 0);
                }


                for (var i = 0; i < MAX_FREQ; i++)
                {
                    var c = e.Result[i];
                    double intensityDB = 8 * Math.Log(Math.Sqrt(c.X * c.X + c.Y * c.Y));
                    double minDB = -96;
                    if (intensityDB < minDB) intensityDB = minDB;
                    double percent = Math.Min(intensityDB / minDB, 1);


                    newData[i / factor] = Math.Max((byte)((1 - percent) * (255)), newData[i / factor]);
                }

                data = newData;
            };

            sampleAggregator.PerformFFT = true;

            // Here you decide what you want to use as the waveIn.
            // There are many options in NAudio and you can use other streams/files.
            // Note that the code varies for each different source.
            var waveIn = new WasapiLoopbackCapture();

            waveIn.DataAvailable += (s, e) => {
                byte[] buffer = e.Buffer;
                int bytesRecorded = e.BytesRecorded;
                int bufferIncrement = waveIn.WaveFormat.BlockAlign;

                for (int index = 0; index < bytesRecorded; index += bufferIncrement)
                {
                    float sample32 = BitConverter.ToSingle(buffer, index);
                    sampleAggregator.Add(sample32);
                }
           };

            waveIn.StartRecording();




            SerialPort port = new SerialPort("COM6", 115200);
            port.Open();
            
            while (true)
            {
                port.Write(data, 0, NUM_LEDS);

                System.Threading.Thread.Sleep(20);
            }
        }

        private static void InputStream_DataAvailable(object sender, WaveInEventArgs e)
        {
            throw new NotImplementedException();
        }
    }
}