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.
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Fronts: 2 Magnepan MG-III bi-amped w/ pair of Threshold S/300 Stasis Linear Amps, fed thru Magnepan XO-1 Crossover (all purchased Jan 1986)
Surrounds and Backs, Center: 4 Magnepan MG-MC1, 1 Magnepan CC2; driven by Parasound HCA-2205A 5-channel amp (all purchased Feb 2002)
Sub: 1 Miller & Kreisel (MK) MX-200 (Feb 2002)
Pre-Pro: Outlaw Audio 975 (2012); replaced Rotel RSP-1066 (2002); replaced Threshold FET One Preamp (1986)
Display: Vizio XVT3D580CM (2012)
DirecTv: HR34-700 (2012)
Blu-Ray: Oppo BDP-103 (2012)
Laser Disk: Pioneer CLD-900 (1986)
Power Control: Panamax MAX-5100, APC Battery Backup