server.js

var sys   = require('sys'),
    spawn = require('child_process').spawn,
    http  = require('http');

http.createServer(function (req, res) {
  var ffmpeg = spawn('ffmpeg', ['-i /path/to/file', '-f mp3', '-']);

  res.writeHead(200, {'Content-Type': 'audio/mpeg'});

  ffmpeg.stdout.addListener('data', function (d) {
    res.write(d);
  });
}).listen(8000);