Any thoughts or consideration of adding a separate copper ground rod for the audio only panel?
Do it, but you MUST bond it to the rest of the grounds. You can't create an independent ground network that is separate from the house ground that is bonded to neutral at the service entrance.
Also, and this is from reading a long long time ago, I have a faint memory that research regarding surge protectors and lightning strikes showed that bare armored ground (BAG) was superior than merely using a ground cable to the panel. Something about the armor carrying high frequency ground currents better. Usually the armor is not required or installed.