Thanks for your good advice. I tried all of your suggested procedures and the symptoms remain unchanged. As I stated in my original post, even if I back the headphone plug out of the jack slightly to get audio from both earpieces, the laptop's internal speakers remain on. I am thinking of buying USB headphones to get around the problem, as they probably cost less than having the headphone jack repaired, but if they don't mute the internal speakers, I will be no better off.
Thanks again,
Leo_M