Hold the power button down until you get the selection screen. Choose "Vibration Only". When you reboot you will get only a quick vibration, not the splash screen sound. You could also choose "Silent Mode" but you won't hear or feel anything if calls or messages come in. It's not a big deal to put the phone on silent or vibrate mode before you boot up and then change it back after boot. After all, how many times a day do you reboot your phone???