creating an audio off button

I did never do this, but you can try to select an empty audio stream with this button. Just link the button to the following script :

mov GPRM 1, SPRM 1 (to store the actuel audio stream for further sound activating)
Set System Stream Au (8 (empty))
Resume

I don't know if this really works.

You might have to have an audio stream with actuall files in it. You could create an audio file that is the same length as your other audio track but have it completely silent.