1. Get a copy of the system.gaiamobile.org application's folder from the system:
Now that you have tha application.zip file, you can act in two different ways:
2a. Replace the STARTUP sound (normally the path is /resources/sounds) with the custom one (it can be a mp3 or an ogg file, depends on the device).
2b. Add your audio file in /resources/sounds (or in another path if your startup sound is into a different folder) and go replace it in /js/init_logo_handler.js (normally this file is responsable of the startup processes).
3. Push the app and its folder on the data partition, exactly in /data/local/webapps using a temporary root access::
4. Get the webapps.json file:
And change the value of the system.gaiamobile.org app
in this way
Use the JSONLint website to verify that the format is correct for any json file you want to modify.
5. Afther this change push the webapps.json file in its place and reboot the phone:
After rebooting you can finally enjoy your custom startup sound. All the changes will be restored after a factory reset or after the update.