It is the way you name your tag that goes wrong.
Try this out:
<img src="https://dl.dropboxusercontent.com/u/2560922/mobiledev/forum/stopSoundOnTag.png" border="0" />
The problem is that your event Audio -> on sprite.sound ended
In the Touched event you have touched a sprite, so it will know what sprite.sound to pick. But in the ended event. Which sprite.sound value should it pick?