This thread has inspired me to check out FoF again. You see, I messed with it a while back and I seem to recall what put me off was that if you missed a note, the song didn't change. Perhaps there was some option that I was missing or something but whatever it's in the past... So now when I'm messing with it for the second time I see that it's programmed to do one of two things, a) cut all sound if you miss a note until you get another or b) play an alternate song file at the same position.
The first of the two is crappy for obvious reasons like you loose the beat of the song. The second isn't quite perfect either the way I've seen it implemented in the few songs I've acquired. You see, there's a .bat file that contains this:
echo off
@echo COPY
for /r %%a in (.) do (
@echo %%~na
cd "%%a"
if not exist song.ogg copy guitar.ogg song.ogg
)
cd..
@echo END COPY
Which seems to just make a copy of the song at a lower volume so when you play the game and miss a note the song just get's quieter. (This is based on what I hear not what the code means, I don't understand that...

) It would be nice if there was an easy way of removing just the guitar, but I imagine that's too much to ask. I have come across a song that implements the switch better Freezepop's Get Ready 2 Rock plays all but the guitar when you miss a note.
Perhaps it's just a matter of looking for better songs/song packs that contain "guitar" and "song" tracks custom made rather than making them yourself with a .bat...