correct, however, he was correct in saying they are an extra drain on the alternator. Once they are used up, they have to be recharged. Best bet is to charge them up every night and keep an eye on your voltage while playing. If your voltage starts dropping low and stays there, cut off the music and let stuff catch back up. Also, no need to run the stereo with the alternator not running.
I also read up, as you stated above I will need to charge my batteries.. is there a difference between playing until low voltage.. (which would mean batteries are pretty drained) and then just turning my music down/off so the alt can charge them rather than having to take them out and charge them?
from what ive read, yes, there is a difference, alternators to not provide a full charge like a charger does, at night, or at least once a week hook them up to a charger, an alternator isnt a perfect substitute for a charger, it just helps maintain their resting voltage. (if im wrong, feel free to correct me, as im learning some too)