index.js

var Transform = require('stream').Transform;
var inherits = require('inherits');

function Volume (volume) {
  if (!(this instanceof Volume)) return new Volume(volume);
  
  if (volume === undefined) volume = 1;
  this.setVolume(volume);
  Transform.call(this);
};
inherits(Volume, Transform);

Volume.prototype.setVolume = function(volume) {
  this.volume = volume;
  // c.f. https://dsp.stackexchange.com/questions/2990/how-to-change-volume-of-a-pcm-16-bit-signed-audio/2996#2996
  //this.multiplier = Math.pow(10, (-48 + 54*this.volume)/20);

  // c.f. http://www.ypass.net/blog/2010/01/pcm-audio-part-3-basic-audio-effects-volume-control/
  this.multiplier = Math.tan(this.volume);
};

Volume.prototype._transform = function(buf, encoding, callback) {
  // create a new Buffer for the transformed data
  var out = new Buffer(buf.length);

  // Iterate the 16bit chunks
  for (i = 0; i < buf.length; i+=2) {
    // read Int16, multiply with volume multiplier and round down
    var uint = Math.floor(this.volume*buf.readInt16LE(i));

    // higher/lower values exceed 16bit
    uint = Math.min(32767, uint);
    uint = Math.max(-32767, uint);

    // write those 2 bytes into the other buffer
    out.writeInt16LE(uint, i);
  }

  // return the buffer with the changed values
  this.push(out);
  callback();
};

module.exports = Volume;