hmmm… the only ideas for the moment are then to start MSFS with disconnected decives , try to stop windows audio service while MSFS starts, or if we assume MFSF loaded a former setting from cloud try to remove the local usercfg and start offline… But all of these is a bit… strange 
EDIT: and possible you can try -FastLaunch… showing these useless startup video should not made a difference, but who knows