yeah brewski i've seen that before. Being it was factory installed then there must be a purpose for having the ability to turn it off. Maybe for the maintenace personel, i dont know for sure. The environment the truck runs in determines on how it should be set though. But if it does not have the option to turn it off it should not be disabled.

As for muffling one, covering it up so it's not so loud, anything to alter the natural design of how it operates i do not think is allowed.
they do make sound sensitive alarms that change decibal levels depending on the sound levels around it, i've also seen some that have a hi low switch on them too.

I was told by various factory engineers at different companies when that question arrised and my training teachers that putting tape over one, stuffing paper or anything inside one to muffle it is not acceptable.