If your app uses AVAudioPlayer, you must call prepareToPlay() before the Vivox SDK uses the microphone. If prepareToPlay() is called while the Vivox SDK is using the microphone, the audio for that instance of the AVAudioPlayer will be at a lower volume. For more information, refer to the Apple developer documentation on prepareToPlay().

This behavior can also occur when recovering from an audio interruption. For a workaround, see Restore audio volume.