I'm finding that when clicking on some iAds, and then returning to the game, that sound does not come back. It's specifically the video-based ads that do this. Right now, there is a BMW ad, for example, that causes this.
Nothing I do in the game can bring the sound back. I have volume controls, but they don't restore it. Furthermore, going to the home screen and coming back doesn't restore it.
The only way is to kill the app outright, and relaunch it. This is tedious, and I doubt end-users will bother doing this.
Testing on an iPad.