If you are using a iOS device which has not been updated to iOS 8 you will need to check the microphone settings in a different location on your device.
1. Tap "Settings" on your iPhone.
2. Tap on the "Privacy" option inside settings.
3. Tap "Microphone" inside the Privacy Menu.
4. Make sure the slider is all the way to the right and is colored green. If this slider is to the left tap on it to enable microphone access.