This App uses a workaround to suppress annoying startup sounds on some Android phones. It basically mutes the phone on shutdown and restores the previous state after boot. The later versions include some tweaks to improve reliability, such as a notification to keep the app in memory.
This section addresses some common issues with Silent Boot. The workaround still is pretty unreliable on many phones and I could not find a particular reason why. So many answers are based on user feedback or assumptions.
A: It seems like HTCs phones running Gingerbread use Fast Boot which basically puts the phone into a sleep mode instead of powering it off normally. That is the reason why Silent Boot can not mute the phone or restore the state as it would do on a normal shutdown/boot sequence. Fast Boot can be disabled in the power settings menu.
Q: The latest update broke Silent Boot. Can I downgrade?
A: The download section at the bottom of this page offers current and previous releases. Just pick the top download in the "older files" section (or any other version you may like).
latest test build
You can find the latest project source on github.