I do this quite often. But, my method is not direct, since I'm actually doing more than just converting from one format to the other. What I do is play the .mp3 file using Windows Media Player, and record it into a new stereo track in my sequencer program. By default, the sequencer records as a .wav file. Now, I usually do other processing of the file. Adjusting volume, adding other effects, etc.. But, the process does convert .mp3 to .wav.