It's not just the music player. Not even my ringtones work (if I have one of my custom made MP3 ringtones set, I get a call and it's silent). It seems like my phone has lost the ability to play MP3's entirely. Actually, it has been acting strangely in general - I just tried to call someone and after ringing once, I started getting some weird squealing/hissing/horror-movie like sounds.
Is this MP3 problem a common error in Cognition 3.04? I don't feel like re-flashing if 4.1.1 won't fix it. I've thought also about going to stock Froyo (since it was just released)...I get the feeling there will be fewer bugs.