The SE version of UO uses nothing but MP3 files, replacing them is very simple, just drop in the MP3 folder after naming it the same as the old music file you want to replace.
You cannot replace a .midi file with an MP3 file without editing the client itself and server code which controls the regions.
UO AOS version uses mostly MP3's as well and you can replace those with any other MP3 track you like by simply renaming it the same as the file your replacing in the UO/Music folder.
Furious Angel is a good one for Combat by the way
and I highly recommend playing the ringling brothers theme song for the Britain city track since thats where all the "Clowns" hang out in the online version and act stupid.
Dev