Can you clarify some stuff about the truck not starting (there are various degrees of not starting)
did the starter click?
did the starter spin?
did the starter spin the engine (cranks OK)?
did the starter spin the engine (struggles to crank/cranks slow)?
did the engine just spin or did it chug and try to start?
I would suspect a slow drain on the battery through the amp, when you had it connected the amp drain+starter drain was enough to keep the engine from starting.
for now you might want to get a trickle charger. remember that if you discharge a regular auto battery too low or too many times it deteriorates rapidly.