have been working on the AVMutableComposition to mix audio file with video,
For the part of insert the audio at video time 0, am using this
AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[a_compositionVideoTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
value - as usual, amount of quantums (for example, seconds) timescale - length of this quantum in seconds
CMTimeMake(1,30) // one interval of 30 sec CMTimeMake(30,1) // 30 intervals of 1 sec
In fact it is the same absolute time, But it has different granularity, which is important when you deal with audio and video file processing.