FYI, I’ve tried opening the mixer in mono (1 channel vs. 2) and now the
mixer audio plays a little closer to the right speed. (It still ends
much earlier than the video, but it doesn’t sound like I’m watching the
Chipmunks 
)
I still have the problem with mixer audio closing(?) when the mpeg audio
ends.
(All audio comes back if I restart the mpeg, btw.)
Hope that helps someone. Thanks!
-bill!