They don't. They open the main park at 9am and they just have Mickey come to the turnstyles to welcome people in. The lands don't open until 10am, they're roped off but they just drop the ropes at 10am. There's no ceremony.
It's a similar story in the studios. A character nromally comes to the gates to welcome you. Studio one is the only bit open at first. They do have some music and an announcement as they let you out of studio one.