Thanks again, everyone, for the good suggestions. This computer just doesn't seem to recognize the tv in the sound department. Our other laptop connects fine, and when connected, if you hover the mouse over the speaker icon in the taskbar, it shows the tv's model number. and when you right-click the icon the sound dialog box lists the tv.
To be specific, this is a Dell Inspiron laptop, model 15-5559. The other computer that does connect is a Dell Inspiron laptop, model 13-7347. They both have the same Dell Audio. They both came with Windows 10.
Now here's the really crazy part. Tonight, after connecting the other laptop and verifying that it indeed worked properly and the tv showed up in the sound dialog box, I disconnected it and then connected the problem computer. I was surprised to find that it now had sound on the tv! I disconnected it, and tried connecting it again, and now it's back to not recognizing the tv for audio. Video is fine. Just for the heck of it, I tried reversing the HDMI cable, end for end. No difference. This is weird. I'm trying to be methodical, but wonder what I'm missing...