They rate the amps at 14.4 because they higher the volts = more watts from the amp. Some car audio companies rate their equipment around 12.x volts to make it more realistic.
13.8 is perfectly normal. And I disagree with the seat belt bolt being a bad bolt. With the proper cleaning of it - I couldn't think why it would be bad. I've always done it from there
Also if your alternator was bad you wouldn't be able to drive your car because nothing would be recharging your battery. Check your battery voltage now and see what it says after driving it. Anytime from 12 down is bad imo