AVCaptureSession audio doesn't work for long videos

I found the solution as an answer to a completely different question.

The issue is the movieFragmentInterval property in AVCaptureMovieFileOutput.

The documentation for this property explains what these fragments are:

A QuickTime movie is comprised of media samples and a sample table identifying their location in the file. A movie file without a sample table is unreadable.

In a processed file, the sample table typically appears at the beginning of the file. It may also appear at the end of the file, in which case the header contains a pointer to the sample table at the end. When a new movie file is being recorded, it is not possible to write the sample table since the size of the file is not yet known. Instead, the table is must be written when recording is complete. If no other action is taken, this means that if the recording does not complete successfully (for example, in the event of a crash), the file data is unusable (because there is no sample table). By periodically inserting “movie fragments” into the movie file, the sample table can be built up incrementally. This means that if the file is not written completely, the movie file is still usable (up to the point where the last fragment was written).

It also says:

The default is 10 seconds. Set to kCMTimeInvalid to disable movie fragment writing (not typically recommended).

So for some reason my recording is getting messed up whenever a fragment is written. I just added the line movieFileOutput.movieFragmentInterval = kCMTimeInvalid; (where movieFileOutput is the AVCaptureMovieFileOutput I've added to the AVCaptureSession) to disable fragment writing, and the audio now works.

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.

Updating videoFileOutput.movieFragmentInterval = kCMTimeInvalid solved this for me.

However, I accidentally set the movieFragmentInterval after calling startRecordingToOutputFileURL. An agonizing hour later I realized my mistake. For newbies like me, note this obvious sequence.

videoFileOutput.movieFragmentInterval = kCMTimeInvalid
videoFileOutput.startRecordingToOutputFileURL(filePath, recordingDelegate: recordingDelegate)