Wave and Mp3 sound problems

Erm I am having problems with my flash after when I export the movie. I have reedited my wave and mp3s (trimming them since I need only some of the sound effects in it) But after when i exported it, the sound’s quality doesn’t seem to be as good as before (sounds muffled and a bit blur). So I want to know is there anyway to prevent this? Also is there any recommended freewares to edit trim mp3s? Help would greatly be appreciated.