Russ - I have already uninstalled my sound driver (which automatically reinstalled upon restart), though the issue persists
Puiu - I've tried several different headphone types, including my external mic (which usually will trigger the headphone-speaker mute.
While searching the net for a solution I came across one forum that referenced a similar sound issue with the Acer Aspire laptops, but the reference was one to a person asking if they could use both Jack and speakers at the same time - the opposite of my problem.
Thanks for the thoughts so far...