This just happened again to me, I’ve got my desk speakers plugged into the headphone jack of my LG UltraGear monitor and using a DisplayPort cable to connect it to my PC.
What I did to get my sound back in Windows 11, opened Control Panel and clicked on Sound.
From the sound pop-up, momentarily disabled my default device and then went into the sim and selected headphones instead.
Then I re-enabled my default device and back in the sim settings, reselected my default device and sound came back on.
Hope that helps someone.
Not sure how my sound came to go off again, I started the sim from XBox Game Pass same as normal but the sound wasn’t working and the multiple instances appears under my volume mixer again.