I've brought this up before, but I do maintain an out-of-tree sound driver (multimedia/cx88) that happens to use these version constants; for all I know I may be the only person who still uses it at this point, but I'm the maintainer so that's what matters.
I also don't think it's safe to assume there aren't other such drivers (ISTR there was at least one other one that was related to some graphics kmod).

"Just rebuilding" isn't a viable option for an out-of-tree driver here: You're changing the required version from 5 to 1, so any out-of-tree driver that wants to support multiple FreeBSD versions won't have a clean way to determine which of those versions to use to avoid kmod dependency check failures. The driver could use #ifdef SOUND_MODVER and hardcode 1 otherwise, but that strikes me as a hack. Otherwise, you'd need to bump FreeBSD_version and let drivers check that, but if anything that seems even uglier.

I really don't see why this change is necessary: you're saving a total of 6 lines in sound.h, and those 6 lines don't seem very likely to cause any ongoing maintenance burden. In exchange for that you're causing churn in all in-tree audio drivers and risking breakage of out-of-tree drivers.