CLICK HERE FOR GOLDWAVE
This little prog called Goldwave is very cheap and very versatile. If there is anything you want to do with most supported sound files this is the prog for you.
I was recommended this by another MTU user some time ago for adding silence gaps at the begining and end of wav files for puttuing in Title text and closing credits.
It supports MP3 and converts to wav no problem.
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But remember that your converted MP3 to wav file is not going to be as good a quality as an original wav file taken from a Audio cd. You wouldn't be in such good condition either if someone compressed you to 10% of your original size.