Set your timer widget to be the data source of your text that is the countdown display
Set widget to start when online and to reset when offline
Create a second scene director (not default)
Add your .WAV audio file to the scene director
In visual logic editor, use string compare block to compare text of your countdown display and a string value block set to 0:00
Output the equal node to your scene director.Actions.Play block
This will only work once for each time you put the scene online, (because the visual logic fires when you put the scene online) but you should be able to reset (or stop) if you haven't reached zero and fired the scene director with the sound effect.

#XPression