(safe method)
1. Get a copy of the system.gaiamobile.org application's folder from the system:
adb pull /system/b2g/webapps/system.gaiamobile.org
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::
adb push system.gaiamobile.org /data/local/webapps
4. Get the webapps.json file:
adb pull /data/local/webapps/webapps.json
And change the value of the system.gaiamobile.org app
"basePath": "/system/b2g/webapps",
in this way
"basePath": "/data/local/webapps",
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:
adb push webapps.json /data/local/webapps/
adb reboot
After rebooting you can finally enjoy your custom startup sound. All the changes will be restored after a factory reset or after the update.
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The article above seems to have been written for another phone, since the CAT B35 file structure differs a tiny bit in that it has a folder with /resources/sounds folder with a startup startup sound file, but thats not the sound that im trying to erase from the face of the earth. then there is another folder /resources/power/ which contains the poweroff video and some other things, but no power on. I actually succeeded in removing the CAT logo/video when turning my phone off, which is a minor achievement since I dislike brands on my things, but no matter how hard I look, I dont seem to find where the startup sound/video is stored so I can nuke that file.
I also tried editing /js/init_logo_handler.js in such a way that theres no video or sound whatsoever when powering on and off, I did that by altering most of the variables to NULL. It didnt have an effect, and I know that those variables come into effect when modified because when I try pointing them to, for example, the CLEAN MEMORY.mp4 video that is used for something else, that video is played at startup, but not as a replacement of the CAT with the truck sound but as an follow up to it, making booting a bit longer....
so.. My guess is that Caterpillar REALLY wanted to annoy us when booting our phone, so they went an extra mile while coding this phone and made it very difficult to remove.
if anybody managed or has an idea of what I could try please let me know!