iPhone won't play sound in *some* apps My friend is having a problem with an iPhone 4 (not yet updated to iOS 5).

Most apps and the ring tone will make noise via headphones but not via speaker. iTunes and at least one other app will play sounds via the speaker, however.

What could cause some apps to become muted on the speaker but not others? No settings were deliberately changed, but there are young children who use the device and could have set something. Is there a simple way to reset the phone and preserve apps and data but make the speaker work again in all apps?
The reason why some apps played sound and other didn't is because individual iOS applications can decide whether their audio should play regardless of the mute switch state.

Apple recommend that if audio is critical to the usage of the app, for example a "teach me German" speech app then should probably ignore the state of the mute switch. However if the audio is superfluous to the main function of the application then it usually should not play when the mute setting is on.

Additionally an application can decide to blend its sound with existing iPod music playback, or to pause iPod playback when the application launches and manage sound exclusively by itself.

The mute switch does not affect sound through headphones.