Processing

import processing.sound.*;
Sound s;

void setup() {
  size(200, 200);

  // Play two sine oscillators with slightly different frequencies for a nice "beat".
  SinOsc sin = new SinOsc(this);
  sin.play(200, 0.2);
  sin = new SinOsc(this);
  sin.play(205, 0.2);

  // Create a Sound object for globally controlling the output volume.
  s = new Sound(this);
}

void draw() {
  // Map vertical mouse position to volume.
  float amplitude = map(mouseY, 0, height, 0.4, 0.0);

  // Instead of setting the volume for every oscillator individually, we can just
  // control the overall output volume of the whole Sound library.
  s.volume(amplitude);
}