BMW Repair: Radio/Battery Problem, harmon cardon, carpet panels

Question
I recenty changed the battery (AC Delco brand) in my 2002 325i and now it starts fine but the radio doesn't work. It lights up and allows me to scroll through tracks/stations but doesn't produce any sound (3 times sound has come out over 3 weeks for short burts). Before I changed the battery the back speakers would go on and off and rumble a bit at times.

Answer
Alex sounds like an amplifier issue, If you have Harmon Cardon/Top Hifi option then see if you can find someone with the same Amp (Left rear corner of trunk area behind carpet panels. Usually black and has a lot of heat sink fins on it. Anyway try to swap it with a buddy and see if that the issue. If not then the Radio may be the issue.
-Evan