This problem seems to be in the official firmware as well as the OpenGD77. I experienced it yesterday and switching to the VFO and back fixed it.
At the moment I don't know what causes it, and its difficult to track down as it doesnt happen very often.
I suspect its a problem with the audio buffer where possibly a frame of DMR data has been lost in the network and this will cause the output audio to be constantly short of 60ms worth of audio.
I think I'll need to put in some debugging to capture if there are any missing frames, but unless this happens when I've actually got the buffer running in the radio, I'm still not going to be able to see if thats the problem.