It looks like iOS 7 is employing more privacy measures than previous measures. On top of being able to block Phone and FaceTime calls as well as text messages, all of your apps will now have to ask for permission to access the microphone in iOS 7.
With this new feature, you can rest assured that your phone isn’t recording your every move and sending data off to who knows where without permission.
While Apple didn’t mention this at yesterday’s WWDC keynote, asking permission to use the microphone will likely work in the same way as when apps ask permission to use location data. Each app will probably only have to ask once.