To follow up:

I have figured out what caused the bug. I had the audio delay set to 2 frames of video. Changing this back to 0 frames made the bug go away. It is easier to live with some minor lip sync problems than to have to continually hit the Mute button.

Another bug seems to be that you can't set the audio delay individually by the source. Contrary to what you would expect in the Meridian software, it seems to be a global value.

Chris Cooper