transcode stream.

transcode.js

var PassThrough = require('stream').PassThrough
	  , util = require('util')
	  , spawn = require('child_process').spawn
	  , os = require('os');

	function Transcoder() {
	  PassThrough.call(this);

	  this.on('pipe', function(inStream) {
	    // inStream.unpipe(this)
	    this.inStream = inStream;
	  });

		this.on('close', function() {
			this.child.kill();
		})
	}
	util.inherits(Transcoder, PassThrough);

	Transcoder.prototype.pipe = function(outStream) {
		// -nostats
	  var child = this.child = spawn('ffmpeg', '-i pipe:0 -f matroska -c:v copy -c:a mp3 pipe:1'.split(' '), {
	    cwd: os.tmpdir()
	  })

	  child.on('exit', function(code) {
	    if (!code) {
	      this.emit('finish');
	    }
	    else {
	      this.emit('error', new Error('FFmpeg error'));
	    }
	  }.bind(this));

	  this.inStream.pipe(child.stdin);
	  child.stdout.pipe(outStream);
	  child.stderr.pipe(process.stdout);
	}