BMW Repair: no sound from speakers 99 E38, parking sensors, nav system

Question
hi, I have an MK2 navigation system in my car and ever since I purchased the car (2 years ago) there has been a slight problem with the system. The system will not play sound from radio/cd/tape and will go into main menu and then back to what it was doing; this lasts for only a couple of seconds therefore it does not bother me and the frequency of which it would happen at each journey depended on how cold it is outside (the colder the more frequent). Lately though no sound will come out of the speakers no matter what I do (usually when i start the car) and can last up to 30min with the screen not showing any problems. Sound will eventually come up again if that "hikup" of the system happens as I described before or just by itself after some time if the "hikup" does not occur. Parking sensors sound even if no sound is coming out of the navi. system.
Im not sure wheather its the amp, wire harness or the actual MK2 navi.
p.s. nav system and video display work finee38
Thanks.  

Answer
Michael, most problems that I run into on these cars is infact the AMP. I am not sure if the AMP is reponsible for prioritizing audio signals like the PDC. I would swap an amp from a buddy first thought just because I've seen the most issue with it. I doubt the Nav. is the culprit.
-Evan