Good news, managed to solve my no-sound from the left speaker problem.

It was an accidental discovery. I moved my right speaker and it shut down my PC. Investigated and thought it was the 3.5 mm plug to my LG monitor (PC to monitor via HDMI), then to the subwoofer box, and the 2 speakers. Possible short circuit when moving the speaker, shutting down the PC.

That 3.5 mm plug did not plug in tightly. Maybe it is a 3.0 mm plug? Or maybe from the past 20 yrs of use, tear and wear?

Using wired headphones to LG monitor, can get both left and right sound. So the problem is not from the PC.

Bought a cable with 2 x 3.5 mm plugs. Cut and reconnected the tiny wires. Bingo, music on both speakers now!

I was afraid the problem was in the PC motherboard or the subwoofer electronic. That will be outside my DIY knowledge and skills.

Finally.