This probably isn't an answer to your question, but from what I understand, USF conversion is pretty difficult because the way the audio works for the N64 is pretty strange. I personally can't explain it, but from what I know, the N64 mixes everything together internally before actually sending it to the player. As far as playing audio straight from a z64 is concerned, there isn't any way to do this as far as I'm aware. However, it is possible to rip MIDI+SF2 from most N64 games. Since yours is a modified ROM, it might not work, but you can look more into it here: https://www.hcs64.com/mboard/forum.php?showthread=45098
I'm looking simply to get WAV recordings of the songs in my rom (I have edited the songs). I want the rom to be played in original hardware or in an emulator - so sf2/midi is out of the question.
Someone elsewhere has suggested putting the songs into a different location in the game so they can be played without other game ambience, etc.