Im not trying to argue with you or anything but it is possible for AC to go from your battery through your power wire to your amps.
the alternator converts A.C Voltage into D.C and its not uncommon to have AC ripples come from the alternator to the battery. This can happen when a Rectifier Diode goes bad on an alternator. New batteries filter out allot of these ripples but if the battery is old it wont be capable of filtering out as much. The AC ripples that go through to your amplifier can cause distortion.
but because the OP said what he was hearing was music and not distortion it isnt AC ripples going through his power wire.