I tried the advanced settings first (easiest!), but that didn't work. Windows has updated three times in two days for me. Every time it does, something new breaks. Usually my browser cache clears out in an unhelpful way, and I lose Chrome logins. This time, it was everything running at half volume.
I'm trying to d/l all the drivers, but I don't know whether to get the motherboard manufacturer's latest drivers or, as someone suggested here, choose Win10 drivers, or even revert to Win10.
[EDIT] I d/l my headphone's updater and found that Sennheiser only supports Win10. The weird thing is that only the latest Win11 upgrade, this week, tanked the volume. The hp were OK with the earlier Win11 updates.