The idea of the audio issue being a seperate problem sounded like a good idea. I turned Dolby Digital off and found that it didn't help.
A bit later, the 3rd grade instructor said she DID get video OR audio. not both simultaneously, to reflect later yesterday. That tends to tell me that it isn't a video only problem.
Again, any more ideas would be greatly appreciated.
Where I run into problems is I am a former Netware 6.1 and Windows Server 2000 administrator (with limited Cisco router experience), whose Apple experience was basically limited to Mac client connectivity. I was out of IT work for some time (2003-2013); when I returned, I had to teach myself Open Directory, update myself in Active Directory, get semiskilled with Wi-Fi, and learn to use "gadgets" like iPads. In addition I taught myself to use Apple Configurator/VPP. I always have to ask myself if my relatively minimal Apple product experience is a negative factor when working with Apple products. I do think my basic network knowledge is solid enough. This is why I appreciate any help I can get with Apple products.