If you have your music as an MP3 or WAV or whatever, then convert it to 44,1 kHz stereo OGG. If it's already in OGG or ACM format, there's no need to convert it.
It's a bit more tricky if you're trying to add new combat music, 'cause that needs to be split up into looping track parts first like the vanilla combat tracks, but this is not EE-specific.
Comments
Everything described here: http://iesdp.gibberlings3.net/file_formats/ie_formats/mus.htm
is still true, except the ACM files referenced from MUS may be either ACM or OGG files with .acm extension.
It's a bit more tricky if you're trying to add new combat music, 'cause that needs to be split up into looping track parts first like the vanilla combat tracks, but this is not EE-specific.