very strange problem with cd and dvd play

Hi everyone, I'm Edoardo from Italy and this is my first post. I'm a professional ICE installer, I don't do a lot of car stereo but I install navigation systems and stuff like that (most of which is Chinese...). I'm having a very strange problem with one of those aftermarket special DVD GPS. When you play a CD or a DVD, seems as everyone inhaled Helium from a balloon before singing or speaking. If you watch a movie, seems that have been dubbed by children! If you play Metallica, seems that Mickey Mouse is the singer. But if you play radio, iPod or MP3 on the SD card it's all fine. I've opened it but to be honest, I really don't know where to look..! Anyone has an idea?

Sounds like a Chinese nav unit to me.