Code:
function intro()
{
  var handle;
  handle = media_play("moonlight.mid",null,100);
  if (media_playing(handle) != 0)
  {
    error("The file is playing but cant be heard.");
  }
}

function main()
{
intro(); // I used snd_play to plat a wav before)
}

you forgot to define the variable.

Always learn from history, to be sure you make the same mistakes again...