Do you know what your initial import settings for the sounds were? The default compression rate is pretty bad, imo. It's made way worse if you are using sounds that were already compressed - it has a sort of exponential effect. That's the only idea I can think of.
It's very hard to get responses around here to audio questions for some reason. I've had several that no one has been willing to take a stab at no matter how many times I bump it. Good luck.