actualy m4p to wav. I dont know if my phone plays mp3 actualy, but even if i can get mp4 to mp3 i can always to go wav from there easily. So any of them is good.
download any half-decent audio program (i like GoldWave), set the input recording device to "What-U-Hear" or "Speaker Output" or the like and play the song and hit record. Save as .wav
iTunes will do it, unless the song has DRM protection. Go to the preferences and select WAV encoder, and then click the song and select "Convert Selection to WAV" from the "Advanced" menu.