I appreciate facts added to the discussion. The possibility that echo suppression is interfering with a Windows 10 setting the speakers at 5.1 Surround sounds plausible and I appreciate that the logic of 5.1 echo suppression may be, shall we say, a bit past the point of economic return on investment compared with tried and tested Stereo audio suppression. Stick with Stereo for Skype.
But Skype could easily determine if the sound environment it finds itself in works. We know 5.1 does not work with Skype, and so must the Skype technicians. So put in the error message. I am interested to see how Teams deals with this issue. Sooner or later there will be a fix or an error message.