Update... looks like Mutex gets stuck forever unless you stop all sound playing on all sound banks, even if the bank you are unloading isn't playing any audio... that is a bug perhaps.
So call AkSoundEngine.StopAll(); before calling bankVO.UnloadBank()
Second problem now... When I call unload, it says there are 2 refs to the bank, so it removes 1 ref, and still does not unload it. I only have one ref / one AkBank script with that bank, so not sure where the other ref is coming from. getting closer...