Lost HDMI Sound after OS Restore

I have an TC-605 desktop which originally came with Windows 8.1.

After updgrading to Win 10, I experienced a lot of troubles with a couple of programs, so decided to go back to 8.1. Fouled up several things and had to order a restore thumb drive.

The restore went fine and I reinstalled the drivers for 8.1 64-bit from the Acer website. However, when all was done, I had no sound.

Checked the Device Manager and found two entries for High Definition Audio - one was OK, the other had the deadly exclamation point. Talked to Customer (lack of) Service and got no where. I even tried an RCA jack between the back of the computer (the red X over the speaker icon went away) to my monitor (AOC 27 inch LED), but still no sound.

The monitor is connected via the HDMI port. Is there a driver for this port/function/whatever? I just can't figure out how to get sound back! Except for the startup bing, I don't even get any sounds from the PC's internal speaker.