I've discovered a simple problem and repair to the speakers on the 850. The magnets fall off of the dash and rear deck speakers, resulting in no sound from the speaker. In the case of the rear deck speakers, the magnet falls off of the tweater, then rests on top of the voice coil of the bass speaker unit.

Whoever made these speakers for Volvo did not count on the effects of heat on the adhesive used to attach the magnets to their plastic base. If you hear a rattling sound from the front dash or there's no sound coming from the dash or rear deck speakers, it's possible the magnets fell.

They can be reattached with any quality adhesive designed designed for metal to plastic adhesion. I was able to repair both the front and rear successfully.