问题
I run the export on the simulator and everything works great. I run it on the device and the video gets exported but there's no audio. This leads me to believe that I must be using an audio format that the device doesn't support but OS X does, as the simulator uses what OS X uses. I've tried m4a, aiff, and aifc and have had no luck! Any ideas??
回答1:
I have a very similar problem. It does not seem to do with codecs, as I made a separate test case that runs fine with the same video. There’s a related question that says the problem might be in playing the same assets using MPMoviePlayerController. That got me on the right track (sort of).
In my case the trouble stem from using the assets in an AVPlayer during the export. I was not able to find the exact combination that causes the export to drop the audio track – in the separate test project the export runs fine even though the asset plays in AVPlayer at the same time. After several hours of trying to find the exact cause I gave up and simply popped the asset out of the player using replaceCurrentItemWithPlayerItem:nil during export. It’s a hack, but it works.
AVFoundation is a very powerful framework, but God I wish it wasn’t so finicky or at least logged more errors instead of silently producing garbage.
来源：https://stackoverflow.com/questions/4911194/avassetexportsession-missing-audio-track-when-exporting-on-device