Hi Antrobus,
Welcome to the forum. Your problem should be pretty easy to solve. When the idle timeout triggers it should generate an event: vc.idletimeout that you can see in the history window.
You can create a command that performs the soft unmute, and then drag this event from the history window onto the new command. That should do it.
Or you can copy the following code, then right click your new command in the tree and paste.
<event>vc.idletimeout</event>