Null object to toggle volume in After Effects?
Add a Slider to your Null Object.
Alt-click on the stopwatch next to the one of the volume controls that you want to control.


This will open the Expressions box for the volume control, and a pick-whip icon will appear.


Drag from the pick-whip to the Slider in your Null.


Some code will appear in the Expressions box, something like:

temp = thisComp.layer("Null 1").effect("Slider Control")("Slider");
[temp, temp]


The numbers on the volume control will turn pink to indicate that they are being controlled by an expression.


Now move the slider on the null, and the volume should change on the audio track.


To do this to multiple audio tracks, just follow the same process.

Here is a screen shot to illustrate this: