Super would do all those things - I don't have any link to it other than I think it's useful

The reason you might want to go from an mp3 to a wav file is to burn it to a CD that would play in a CD player rather than an mp3 player

I recorded a gig we played on friday on my Zoom recorder as mp3 files but wanted to give a CD to one of the others in the band so I changed them to WAV files and burnt them to a CD

You can choose how many kbps you encode your mp3 or you can change them from one to another