batterys dont usually die unless they are quite old or put under constant strain using amps / alarms as the battery hasn't time to charge up due to the amp if working all the time then u got your alarm draining when u car is switched off
must say headlights and amps really do suck power
yours sounds more like the alternator isn't earthed properly or it is over charging or simply knackered
u should get around14 volts from the battery when you switch your engine on from cold and around 13 volts when warm and should sit aorund12 volts when the engine is off
if your getting less then those reference voltages when the engine is switched on then consider your amp shagged as even if your battery is fubar you should get the voltage from the amp whilst the car is running, [presuming it is earthed oki],
i have jumped started a car with a knackered battery and it ran fine until I had to start her up again