My car audio sounds like it's sitting in a bathtub.?

I drive a '95 Nissan Altima. Something's wrong with my speakers--I have a cette deck that I plug a cette adapter into for my mp3 player but the songs sound tinny and echoey, like they're being performed in a bathroom. I've fiddled with the b and treble, but that hasn't made any difference. The radio sounds normal, it's just the cette deck. I've tried it with two different adapters with the same sound result. Is the deck just fried? Is there any way to fix the sound?