idle voltage doesn't mean much for audio or starting power. (trust me on this one, I fix industrial batteries for a living) Unfortunately there are no simple measuring tools available to figure out if you're over taxing the battery. As a general rule the more battery weight you have the better, also some batteries are better at rapid discharge than others. I'm a big fan of the 'pure lead' batteries because their shorted discharge rates are scary compared to old school deep cycles. Its all a matter of surface area when it comes to rapid discharge, so parallel two batteries and you have twice the surface area, switch to an optima and you get more than that.
btw, battery on charge/finish rate voltage has nothing to do with available power. Its about the lead plate composition and charge/alternator voltage, all though higher is generally better...