[chibill]

3. add an option that allows people to permanently turn it off clientside (? possible addon limitation)

+Rep
This is one of the very few times where we actually may be able to do something here. It would take some intervention from Dink to add a custom command, but I'll mention this to @chibill who has recently become our brand new in-house servant I mean programmer for all things serverside.

May I suggest, in the meantime, make a key bind to console 
 stopsound? I'm sure there's many useless keys you can use for that, such as the left click or space bar buttons.

This is definitely do able, I will have to look into how to save data somewhere that survives map changes, (since that causes addons to be reloaded.) But I am sure there is a part of ULX that has a utility for this.

Just looked into it add it will be pretty simple to do, in theory.

EDIT: Just hammered out this feature before heading to bed for the night, going to look over it in the morning before I pass it on to the appropriate people to get it on the test server.