You really don't want to. Whenever you convert from one compressed audio format to a different compressed audio format, it sounds terrible. Because these formats are all lossy (it throws away sounds that it doesn't think you'll notice) you can never get the exact original back. They each use different formulas to determine what to throw away, and what to keep so you won't notice. But, if you say, try to convert wma to mp3, mp3 will still try to throw away what it thinks it should, and that mixed with what wma already threw away, makes for a really really crappy sound.
Brian