from https://www.commandlinefu.com/commands/using/aplay

// run.sh
#!/bin/bash

gcc -x c main.c

# aplay's -d option specifies duration. So even though main.c
# is an infinite loop, it'll kill it after 2 seconds.
./a.out < /dev/urandom | aplay -d 4

// main.c
#include <unistd.h>
#include <stdio.h>
#include <stdint.h>

#define FREQ 220

// 16000 = 16khz comes from the duration (2 seconds).
// Note that aplay will eat bytes at 8khz by default.
// This means if FREQ = 1, it's just outputing from /dev/urandom.

// Probably gets 220 from here: https://en.wikipedia.org/wiki/Piano_key_frequencies
int size = 16000/FREQ;

void main(){
   int i = 0;
   uint8_t v[size];
   read(STDIN_FILENO,v,size); // write "size" bytes from stdin to v.

   // v now contains random data.

   while(1){
      // This algorithm basically averages out the random buffer.
      // Over time, it'll reach a steady state (this produces the fade out effect).
      // How long this takes depends on "size".
      // size = O(1/FREQ), and a large buffer takes longer to reach steady state.
      // this is because freq = 1/wavelength, so our random buffer represents our wave. (I THINK.)

      // Notice that we don't have to generate some kind of sine wave.

      // This sets the index BEFORE the one we output.
      // So the first run outputs directly from /dev/urandom.
      v[i%size] = (v[i%size]+v[++i%size]) / 2;

      putchar(v[i%size]);
   }

}