Set sound to off at start of course

Hi

I am working on a course where I am using a hidden button on the first slide which sets a variable screenreader to true. I can then use this variable to branch conditionally around content that is not accessible and serve up alternative slide(s).

This all works great but I would like to use this variable to mute all audio if the user is using a screen reader.  I have two triggers on the slide master, the first changes the state of the Audio button to disabled if the variable is true and the second executes the following javascript to toggle the sound off.

var appState = require("helpers/appState");
appState.onToggleVolume();//mute 

The button state changes ok but the audio still plays. Has anyone got any ideas?