Aha!

From: http://artofhacking.com/files/TVMODES.HTM

ZENITH:

I think this might work on Goldstar TVs as well, since both Zenith and
Goldstar are the same company, LG Electronics.

Hold the MENU button. The menu will come up and then after a few
seconds will go away. Now let go of the MENU button and enter

9 8 7 6 Enter

You will get a service menu that varies widely depending on model, so
I won't include a walkthrough here. The ones I tried were fairly
self-explanatory.

Problem solved. Went into the service menu, poked around some and changed a setting named "C. Type" to 3 (likely anything from 2-5 would work - changing from 0 to 1 gave me stereo sound, then I got playing - set it to 5, etc). I also enabled the stereo/SAP (a default choice, I think, for the audio) and the audio mode setup option (was previously missing from the menu). While at it, I adjusted the picture size so things aren't running off the bottom, etc. Looks like a former owner went in there and screwed it up.

I'd be interested in a more thorough accounting of the service menu options. There are things in there like "3.58 T", "WhComp" (white compensation?), "6 Keys", "Scroll" and "5 Jacks" (which I couldn't seem to set) that I'm curious about.

After changing the options in the service menu, I had to rescan the channels before I could power the TV off.

I can't select 0 (or 1 or 2) to tune to the video input, but using the channel up/dn button lets me come round to it - makes me happy!

Thanks for replying to my original request folks - I appreciate the helpful thoughts.