- Put your phone into recovery
- Get a working adb shell
mv /system/customize/resource/android_audio.mp3 /system/customize/resource/android_audio.mp3.nothanks
mv /system/media/android_audio.mp3 /system/media/android_audio.mp3.nothanks
I Google'd for quite a while with no luck, but sorry if this has been posted somewhere. I especially didn't want to change ROMs just to remove the sound. I also noticed that in both locations, bootanimation.zip existed which contains the frame-by-frame sequence for the boot up screen.