MP3 to WAV — Compressed → uncompressed for editing / production
MP3 → WAV is the conversion audio editors want — DAWs (Logic, Ableton, Pro Tools, GarageBand) all work in WAV natively, and importing MP3 forces an internal decode. Note: converting a lossy MP3 to WAV does NOT recover the audio quality lost in the original compression. You get a large file that's still limited by the source MP3's quality. WAV is 10-15× larger than the MP3.