I'm using AVCaptureSession to record a video with audio. Everything seems to work properly for short videos, but for some reason, if I record a video that is longer than about 12 seconds, the audio doesn't work.
We also experienced this issue. Basically disabling movie fragment writing will work but it doesn't actually explain the issue. Most likely you are recording to an output file using a file extension that does not support this feature, like mp4. If you pass an output file with the extension mov you should have no issues using movie fragment writing and the output file will have audio.