HDMI sound output thesteriously no longer works in Mac OS X 10.10 (Yosemite)

Everything was working fine with HDMI sound working as it should in Mac OS X 10.10 (Yosemite). Then recently:

• I turned off the laptop.
• 24 hours later I turn on the laptop
• Connected it to the HDTV via the HDMI adapter + HDMI cable.
• Select “Turn Display Mirroring On.”
• Run iTunes and open a movie.
• Movie is mirrored on HDTV but there is no sound anymore.

This was the exact sequence of steps with no system config changes, no weird download/install, no screwing around with system files. System info screenshots at the bottom of this question.

I tried to play around with the Audio MIDI Setup to no avail. I also tried different Mac OS X reset/boots to no avail as well. This happened last year once already, andI went through all the various solutions from Apple support, including a full OS re-install, all of which didn’t solve the problem.

And yes, I should have written down what fixed it when it happened last year, but I didn’t and now I forgot.

Does anyone have any idea how to delete the HDMI device that appears in “Settings,” so as to get Mac OS X to hopefully properly reset it?

Just throwing this out there, not sure that’s the solution, but can’t think of anything else.

Built-in speakers work fine but headphone output is all screwed up as well, as in different audio channels are all out of mix.