There may be a capacitor somewhere in your sound system that is continually dischargeing even after the the alternator has stopped spinning. As you know the alternator's potential is higher in order to keep the battery charged.

Alternator = 14.5 volts
Capacitor charges to 14.5 volts
Capacitor discharge potential = 14.5 volts

I don't think this has anything to do with your problem.