update: on april 22nd, 2007, new sound mode was added. Using the value '2' (instead of '1' to request sound) will make the server send background music information.
To complement, a new map field, 'background_music', was added. It's a free-form field.
Suggestion for music:
Possibly use a field in the map header to determine if the file containing the music should be sent in the background, so it can be on the player's machine, before the client needs it? – techolous