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Obviously you'll be loosing a lot of quality, but WinAmp used to let you extract an mp3 out to a wav, and configure options for that wav file. Nero's Wave Editor should do it too.
keep in mind, WAV is not a compressed format like MPG, MP3 and WMV are. As Deacon said, you'd be losing a tremendous amount of quality if you try to dissect an already compressed mp3 into an uncompressed WAV which should logically be bigger than mp3 in the first place.
As mp3 typically works on a 10 to 1 ratio (the WAV file turned into an mp3 is typically 10x smaller as an mp3 file since it's compressed), turning a 5MB mp3 file (which should have been the result of encoding a ~50MB WAV file) into a 500KB WAV file (going the opposite way meaning 10x smaller than the mp3 itself) you're going to end up with a WAV file that's probably 8 kHz sampling rate and 8 bit sample size (as opposed to the CD standard of 44.1 kHz samping rate with 16 bit sample).
In other words, it'll sound worse than running the original WAV file through tin cans over a piece of string then sending that sound over regular phone lines.
Bad idea, as already noted above. Perhaps you're getting the formats mixed up, or you just typed the wrong filesize, but for a given quality, mp3 files should be ~10x smaller than the original WAV file used to create them.