it will play MP3's that are burned onto a CD, not a DVD through the nav unit. I am not sure if it will play an audio DVD through the rear entertainment system though. I am sure one of these other guys who know a lot more than me will jump in and clarify that.
The DVD slot in the NAV unit will not play MP3s. You can put a data CD into the upper CD slots and play MP3 files just fine. You can also use a data DVD with MP3 files in the rear entertainment system's DVD player. The only mods I've ever seen for the head unit allow you to feed external (or rear DVD) video into it, NOT play DVDs through it's NAV disc drive.