Some sounds not working

Suddenly, without installing any new apps or doing anything differently, "some" of the sounds on my wife's iPhone 4S stopped working. Incoming calls, text messages, and app notifications from the Notifications Center all work OK. But several app-specific sounds no longer work. Examples are Google search, when clicking on the microphone used to produce a loud sound indicating to start talking, and within Words with Friends, moving tiles around from the rack to the board, etc....all these just stopped working. Several other sounds that used to work "within" apps, just stopped, and it's happening in many different apps so it's not the apps themselves causing the problem. Thanks for any help.