Okay I found the audio file for you, didn't do it last night because of other things.
It's located here: /system/app/SemcCameraUI.apk/res/raw/start_up_notifier.wav
I assume you can delete it and you shouldn't have the start up sound anymore. The problem at the moment is that unlocking the bootloader breaks the camera, and I don't feel like breaking my camera to try and see if it works.
Just to be save, make sure you backup your "SemcCameraUI.apk" file before modifying it and if things don't do as planned, put it back.
"However, if you put any trust in Benchmark Scores you could use them
to prove that dancing naked for 5 minutes in your garden affects device performance" - Chainfire