battery problems

[ppuaar_polosport]

a while ago i was siting outside my mates house with the radio on without the ignition on forabout an hour and my battery died. figures lol wasent the best of my ideas

had to push start it because there wer no jump leads avail

the car has started fine since then. and then today went to go out and tried it and nothing. woul not start.
i have a sub connected to the battery, but this hasnever caused probs becasue it is only on when the car is on right ?

any i deas on what this could be
NEW battery ??
starter motor??
alternator ??

any advice woluld be much apprechated
thanks
Pard

[Adam-]

It won't be the alternator because it is charging the old battery when it has died, and it wont be the starter because that still works?

It'll be the battery, and if its the same age as the car, it'll need replaced!