This is an easy one... there are literally dozens of free applications available to convert MP3 files into WAV files. Actually when you use some CD burning software that allows you to create an audio CD from MP3 files, then that program just did the conversion. Audio CD music files are in WAV format.
I used to use the free version of MusicMatch Jukebox, which was a good MP3 and audio player AND had the menu options to convert MP3 to WAV and WAV to MP3.
I currently use Sony's SoundForge - AWESOME program. I don't know what "reason" is, but I'm assuming it is some type of audio editing program. SoundForge works REAL good for editing audio files. You can stretch out the sound waves (visual) to do very fine tune editing that is seamless... I've cut out instrumental pieces from a song, looped them seamlessly, lowered the volume at certain points (where it got too loud), then seamlessly joined them with another "perfect cut" of another instrumental part of that same song. The resulting custom music piece I made for a friend, who gave me instructions along the way as to what they wanted (volume reduction, etc.) and she ended up using it as her wedding march music last year.
SoundForge is NOT free, at least not LEGALLY free, but when you combine it's perfect audio cuts (samples) with Sony's AcidPro (loop player that can incorporate all the cuts and loops you made with SoundForge and actually compose a complete song.... drum parts from this... horns from that... piano from here, bass from there... etc.
Perhaps "reason" does all of this?
Try this free MP3 to WAV converter. It is from a reliable source and should be "clean" (no spyware, adware, malware, etc.).
http://www.mp3fe.com/main_mp32wav.htm