garbled mp3s

I find a lot of the mp3s I download are nothing more than this strange rhythmic static sound. Sometimes those kind give me a message about corruption, and I don't bother downloading them, but sometimes LimeWire doesn't seem to catch them. Anyway, I'm just wondering what causes them to sound that way? I've had files from many different sources that all end up sounding exactly the same way. Even files with a good rating, that several people seem to have, and which are the exact length of the real song. Has anyone else come upon this problem?