Another update: I just tried out Goob's new build, and still no audio in-mission. I did experience something that just adds to the puzzle, though. I was testing this new build out on one of the X missions that uses one of the battle themes as briefing music. I'm almost completely positive that the track that was playing was the high-quality .ogg file, not the low-quality original .wav, which would mean that the mv_music.vp file was at least being partially detected. It seems as though it's only the music in-mission that isn't being loaded. I'm just ditching the music VP entirely for now; I survived on low-quality music with FS1 and 2 for years without complaint.

Just out of curiosity, are Tahna and I the only people who are having this problem?
Edit: This is just strange. Even with the mv_music.vp deleted, I'm still not getting any music. There's nothing else there to interfere with the music; what the heck is going on? The only thing I could possibly think of is that I have the launcher set to use a certain mod folder with the FS2 movies in it; there's nothing else in that folder, though, so shouldn't the music VP in the main FS2 directory still play fine?