when you play with audio - playback is handled totally by quicktime.
so whatever happens with what you see if something quicktime is doing.
and audio takes priority.
quicktime player doesnt have to handle lots of layers and lots of fx...so it doesnt have so much to do. nor does it have to handle interfaces to lots of equipment like video input simulaneously.
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what size and frequency are all your display screens set to?
what is your screen setup?
do you see this when you dont have audio?
can you send me your pdf profile