Curiouser and curiouser...
I have been browsing Si with the same FF instance, on the same machine, for several years, and there are a number of add-ons installed. I tried disabling those add-ons, with the same result (6 hour cookie for AudioOn). Other SI cookies have "reasonable" looking expiration dates, e.g. 2018, 2030, etc.
I tried a different FF instance, with no add-ons or plug-ins installed, same result.
I logged in with Chrome, turned off the audio, and still see a 6 hour cookie there.
I tried on a different machine, again Win10 and FF but no add-ons or plug-ins, same 6 hour cookie.
So if other users are not seeing this (admittedly minor issue), it must be something related to either my machine(s) or my account, but I can't figure out what.
For now I have used a cookie editor to bump up the exp date a couple years, I hope that takes care of it.
Thanks for taking the time to reply