I agree with Ted except for one little thing...

As far as I know, the T4 will play a midi file or anything you are doing on the keyboard at the time and record it to a stereo WAV file. It only records in WAV format and not in mp3. I don't think it has a mp3 converter onboard. So, you would have to take the WAV file out of the keyboard via USB stick... convert it on your computer through iTunes or your software of choice. I may be wrong but that's how I did it on the Clavinovas.

You could take an mp3 and play it while playing piano and singing over top and record all of that to a WAV. You would still have to convert it to mp3 in your computer.

On some of the older PSR models, you could convert your voice to MIDI notes but it only worked from a solo/mono source. It was fun to do but not too practical. I think it was on a PSR-2100 or 2000. It was fun to demo.