Update: As of 2/2/21, Blackboard has released a fix for this issue.
There have been a couple of reported cases of audio issues in Collaborate breakout rooms. When this happens, the audio is completely disabled in a subset of breakout room (some but not all rooms will be affected). Blackboard have confirmed that this is a known issue and they are working to resolve it.
In the meantime, they have suggested that if this occurs you move attendees into a new group when the issue occurs.