Secondary audio send issue. At first audio from secondary required a refresh from the secondary side via "check" and "uncheck", "send audio when receiver is online" to the point I pinned "multiplicity audio" to the secondary "notification area" expecting to do this daily.
Try unmuting the sender:
With all that said. I need to have a mouse plugged in at all times to my secondary to wiggle and click every day before I log into both machines or the secondary won't start the process properly. If I manually log into the secondary I have the fun experience initiating the CTRL + ALT + backspace which results in secondary CTRL ALT DEL.
As a test, if you are 'sleeping' the PCs, try having both stay awake but just powering off the monitors:
Stardock Support Manager