Beginning with iOS 7, you can enable or prevent specific apps on your phone from accessing the microphone. Sometimes, when you download an app and access it for the first time, a screen appears that asks you for permission to access the microphone. If you have given an app permission to do so and you want to revoke that permission -- or vice versa -- you can make the change in the iPhone Settings section.
Turn Me On -- Or Off
Go to the Home screen and tap “Settings.” In the pane that appears, locate the Privacy button. Tap it and then tap the “Microphone” button to reveal list of the apps that have requested access to the phone's microphone. If you want to grant permission to an app, move the slider next to the app to the right, revealing a green background on the button. To restrict an app's access to the phone's microphone, flick the button to the left.
There are many reasons to enable or restrict certain actions on the iPhone, including the access to its microphone. For example, if you use your iPhone in crowded locations, you may want to prevent app sounds from coming out of the phone’s speakers when you aren't using headphones.