I'd just like to point out that if you convert an mp3 to wav it will not improve the quality. The quality of a sound follows the quality of the worst recording, in this case mp3. For example you couldn't record a mono record to stereo tape and expect it to suddenly become stereophonic - both left and right channels would still be the same...
Sampling:
You might as well get the cable to go from your soundcard line out to the MPC input. That way whatever you play on your computer can be recorded onto the MPC. No need to worry about DRM. If you can hear it you can record it

You can also record from the radio for all manner of lofi sounds. The Beatles did it, so why can't you?
Look up "number stations" on the internet. If you have a shortwave radio sample these for some real oddness!
You can sample off CDs by playing them in the computer and using the line out cable.
Plug some heaphones into the MPC record input, crank the gain up and one of the earphones will act like a lofi mic.
Vinyl can be an expensive hobby, but it sounds really good. Humans can hear past the snap crackle and pop of old vinyl but not the squelch and tinnyness of a bad mp3.