The Boot Sound
Annoying isn't it. To stop the boot sound playing simply edit /etc/pulse/session. Look near the bottom of the file, you should see something like:
# Load samples load-sample startup /usr/share/openmoko/sounds/startup_openmoko.wav load-sample touchscreen /usr/share/openmoko/sounds/touchscreen_click.wav
To disable the boot sound and/or the touchscreen tap sound simply place a # at the start of the line, ie:
# Load samples #load-sample startup /usr/share/openmoko/sounds/startup_openmoko.wav #load-sample touchscreen /usr/share/openmoko/sounds/touchscreen_click.wav
If you want to change the sound, simply change the .wav file name to something else.