Edit:
At the time I created streaming levels, Navmeshes would not appear unless set to dynamic, rebuild on load, and with an empty navmesh in the persistant map (a workaround suggested by UE4 staff).
Looks like it’s been fixed since then! I just tried setting static navmeshes in the persistant map and it works perfectly!
No Navmesh generation = no audio/grass problems!
Posts above are still valid for anyone requiring dynamic navmeshes, and odd those problems were only introduced in 4.14ish, but it’s good to know dynamic navmeshes aren’t required anymore. 