Is there anyway I can tell if the 975 is actually receiving audio over the ARC?

It displays HDMI ARC when that is enabled, and the audio source, Vizio VXT3D580CM TV is enabled properly as best I can tell, but of course there's no audio.

The TV did the CEC 'device discovery' successfully with the 975, which was part of the process of getting its Audio Control Setting enabled so I could turn it ON.

All the TV settings are as they should be to the extent that the vendor support and their web site info indicates. The HDMI cable is HDMI 1.4a w/ ARC support.

Not sure what else I can check.

So, if anyone has any thoughts, any assistance would be appreciated.

Is the input configured on the 975 for hdmi/arc

Yep. See Page 30 in the manual: Audio Input has to be set to HDMI/ARC

For those displays without ARC, you can do the same thing by routing back a toslink connection to the 975, correct?
Not sure how you would set that up on the menu.

Yes but you would be limited to the lossy codecs over toslink or digital coax.

It can carry 2-ch PCM, too. That's not lossy. But even so, is it really a limitation? Isn't ARC used for DTV audio? Is there something else?

Yea, the 975 input is set to HDMI ARC and the 975 displays HDMI ARC when it's set to that input. So, I can't think of any way to verify either device.

In any case, this is only for feeding audio from the TV when using "online" streaming sources, like Vudu, on the TV. We're using the TOSLINK solution in the meantime for getting audio from the TV back to the 975. I don't think any of those services can stream hi-def audio, but maybe I'm wrong, so I don't think we're losing any audio quality.

We can also use the Boxie Box or Oppo BDP-103 for the streaming anyway, which are both connected to the 975.

Nothing OTA, SAT, or cable is HD audio.
Same goes for internet streaming.

HD Audio is only available on Blurays.

Wouldn't you verify the ARC is working by having thr TV as the only audio source on and sound coming from your HT speakers.

Well, yes, I can verify it doesn't work. 'Verify' was a poor word choice on my part earlier. What I meant was I can't determine which device is or isn't performing its part of the end-to-end ARC process properly.