speakers don't connect to the batteries
unless they ARE ALSO the amplifier ?
speakers Normally connect to the output of your audio device/radio or the output of an amplifier being fed the a audio signal
disconnecting the battery removes AN 'ELECTRICAL NOISE FILTER', from the audio circuit
try the 'stereo' ? with NO 120v input, unplug from shore power, turn off inverter, or generator
does t hum all the time, even when not using any tv/music/radio etc..
then it could be powerline interfernce
the speaker wires acting like an antenna