Update. Getting Super from sourceforge without virus warnings seems to be a problem. Recently I found Handbrake, which offers lot of the functionalities without the tricky website, so I update my recommendation to use this one instead.
It might be a hassle to navigate the website, but I'd recommend converting your audio files using Super. Open the page, go to the bottom and click on "Start downloading Super" 
Then search on the next page for "download Super setup file..." 
Once you install it, you open the program, drag your mp3 files to its window and select the output audio file as the desired WAV. You can let the default values for bitrate and so on. It can convert almost any format, includes all codecs on its own and works really well.