Not possible without root , which is not available
As long as people continue to buy the carrier branded phones it's never going to change. Most people don't care about the boot logo/sounds, carrier junk etc or they even realize how all these works to change.Welcome to Android Central! As BergerKing above me wrote, some of these startup sounds are enough to put you off your lunch. It would be nice if you could have the option to disable or toggle them on or off, without having to root the device. Someday, hopefully!
As long as people continue to buy the carrier branded phones it's never going to change. Most people don't care about the boot logo/sounds, carrier junk etc or they even realize how all these works to change.
So no this is never going to change on carrier branded phones
But atleast we have option to buy the US unlocked models which don't have any of this crap for those who care.