I don't think FSO is doing anything weird sound wise, so that's just strange.
I haven't come up with any ideas for you yet but I'll keep at it, along with some more google searches, and let you know if I find anything.
Thanks. I also believe this is not really a FSO issue, since the same installation with the same files worked fine 1 month ago.
I also tried updating the Realtek drivers to the latest version distributed by Asus for this mainboard, which seems to use a driver (RTKVHD64.sys) from 12/22/2020 (the previous one was from 2016).
This seemed to work fine with OpenAL-Soft (v1.23.1), sound worked reliably without the strange behavior of needing some other application use the sound device.
But updating that driver broke some other things in the system, namely all Chromium-based applications (Chrome, Steam, GOG launcher, Knossos?) being broken or needing 30 seconds to start, so I had to use a system restore to fix that. But it seems the issue can be solved by using latest Realtek drivers with latest OpenAL-Soft.
The Win10 installation I'm using here was upgraded from Win7, so has a lot of old cruft and has developed some other peculiarities. So I think it's ripe for a disk wipe and clean reinstall, which I suspect might fix this issue.
I can rule out any HW issues, since this machine is set up with dual-boot to Linux Mint, where a Knossos-based FSO installation works normally.