>  3. Did I hurt my Aux Battery?

I have a large ham radio and have lots of battery experience, some of the
brands don't last more than a few years.  Here is any easy way to test the
capacity of a smallish battery.  Wire a tail light across it by itself and
time how long the light burns a decent glow.  Since the wattage is on the
bulb you will get a good idea about the condition of the battery.  Power in
Watts = Voltage times current.  6 watt bulb on 12 volts draws a 1/2 amp, 12
watt lamp on 12 volts draws 1 amp.  24 watt lamp on 12 volt draws 2 amps.
A 100 watt sound system full on will draw around 9 amps and more depending
on other features and volume level.  A older 40 amp hour battery might be
good for 2 hours in my opinion.

Try not not to drain any battery below 11.5 volts, that level varies by
battery but its generally accepted as the bare minimum before the battery
plates warp and short together, bad news.

Its possible the battery is ageing and the sound system draws far more power
than estimated.  Put an amp meter in series with the system to measure.

The only sure way to find out the battery chemistry type is to research the
brand/model number. Sometimes a deep cycle/gell or AGM will weigh a few
pounds more than it automotive type cousin.  Well at least you get a rough
idea.  I have the same problem, a 55 amp battery that weighs a ton.