I searched around a bit and haven't found anything mentioning this, so here goes:
Whenever I start to play Take No Prisoners or Can I Play With Madness, the crowd noise starts up (slightly louder than usual) and plays over the now slightly muted music track. After a while, it dies down to the ambient crowd noises (shuffling feet/normal conversations) but the music is noticeably muted and somewhat distorted.
Whether it's just these songs that are affected or some other ones as well, I can't say as I haven't gone through my whole library.
On a somewhat related note, when I played Blitz today, after I would finish a song and get to the Results screen the crowd would begin cheering, eventually dying down. This happened for EVERY song that I played.
Maybe it (the latter issue) will clear up the next time I play. The former issue has been around for a while now...