ANSWER FOUND: Apple Music playing sound but no video.

If you are using Apple Music and trying to play the music videos (of any kind) and only the sound comes through but no video, (but the screen changes size like there should be a video) I found that this is a problem. If you are using any kind of 'screen sharing' the viedo will automatically not play. As soon as I stopped sharing my screen (even though it was for the purpose of having additional monitors) the video started playing again. The video will immediately cut in and out as you toggle the screens on and off to. I hope this helps and someone benefits from this info that I learned today!

2020 iMac Retina 5K, 3.3ghz Intel processor