Why is my car battery dying?

I have a 2004 Honda. I got a beater car for the summer and stopped driving my Honda. It say for a few months, the battery was dead, so I went and bought a new battery. At autozone I was told the battery is only a year old, tried to recharge it but said it was no good. I bought a new battery, drove for two weeks and it crapped out on me. The first sign was that my stereo began to shut of and coming back on. Then when I turned off the car it would not start back up. I know that there is a possibility that my alternator is not charging the battery but my stepdad also said it can be a puncture on one of the cable connected to the battery. I have a sound system, and the fact that my stereo acts up before my battery dies might mean that there is in fact a puncure somewhere in that line. What do you think? Any suggestions will be appreciated. Thanks

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