You can play a .mp3 or a .wav
For an mp3 it will have to take the place of one of the music tracks from the original game, you can read here:
http://twhl.info/wiki.php?id=161For a .wav, you'll want to use an ambient generic and stick to 22,000 HZ 8 bit Mono format:
http://twhl.info/wiki.php?id=122As for length, there may be a limit, I've never tried anything that long. Give it a shot, if there is, you'll have to split it up into smaller chunks.
Something to note - sometimes you can get audio not coming back after loads from saves, so if you have a track that long, depending on your situation, you might run into trouble there.