audio voltage

I know this sounds like a stupid question, but I just want to confirm.... When your car is running, but at idle, the voltage should be anywhere between 13.7 and 14.1? correct? And when your car is off with the key in the accessory position a voltmeter should read between 11.7 and 12.1? correct?

When I drive at night with my stereo cranked up the headlights dim. I went through 1 Optima red top within 4 years of time and just recently replaced it. I was hoping that I didnt have to replace it soon because of the dimming headlights or replace an alternator. I know a simple solution is to just turn down the stereo....but when a good song comes on you gotta rock out to it. After installing my voltmeter gauge today it was between 12.9 and 13.3 depending on the bass notes....and the AC was on because it was hot as hell outside today...but when I turned the AC off and left the stereo at the same volume it stayed near 13.3