Sounds like your right....alternator may be on its way out. But if your sound system is drawing excessive amps, then your alternator may just not be putting out enough. I don't know what kind of autoparts stores you have in your area, but where I live they can usually check out your charging system free. This way you can pinpoint the exact problem. As for changing out your alternator for a higher output, I would just recommend that you make sure it will fit with your engine brackets. Good luck! =D
Its the cats pajamas!