Thanks again for your helpful suggestions. Unfortunately, in the "sound" window of this particular computer, the speakers and headphones are listed as one item, so I can't apply the default setting that you recommended.
Additionally, I have discovered that when I back out the headphone jack to get sound from both earpieces, I get just the left channel from both sides, not stereo.
I'm convinced that the headphone jack must be damaged, so I plan to buy USB headphones.
Hopefully, the internal speakers will turn off when I plug in the USB phones, and I won't have to change any sound settings every time I switch from speakers to headphones.
Sorry about the length of these posts. I'm sacrificing brevity for clarity.
Regards,
Leo_M