One possability is that the monitors are shuffling once you disable it.
So Monitor 1, 2, and 3 (with 3 being the AVR).
You switch the switch on monitor 2, effectively unplugging it, and windows moves monitor 3 to the monitor 2 position. Now monitor 3 doesn't exist and audio stops working.
If you can, I'd move move the SPIDF connector for getting audio from your PC to your receiver. That should resolve this behavior.