UPDATE:
After trying it again, and after checking the "Sound" applet in Control Panel, it's now working for me.
Interestingly, there was a "Digital Audio (HDMI)" listed as a playback device. I thought I'd hook up the Surface again and choose that if it played again through the Surface speakers.
But when I plugged it in again, and then looked at the Sound applet, I actually didn't need to change anything. No, it hadn't chosen the HDMI digital device for playback in my case, rather, it created a new "High Definition Audio Device" -- my Toshiba TV
-- and selected it for playback. And that's exactly what happened -- the sound came through the TV speakers.
I'm still curious how the OS got a driver for my TV. It seems likely that it downloaded it automatically over the Internet. But I have at least confirmed (in my mind) that the best means of troubleshooting this issue is through the Sound applet in Control
Panel on the Desktop.
To get there, go to the Desktop first. The open the charms menu and choose "Settings", and "Control Panel" will then be the 2nd option listed from the top. Once you're in Control Panel, it's practically identical with the Windows Control Panels of yesteryear,
so you should be in familiar territory at that point.
Cheers!