Look at AVFoundation framework ... There're many ways, but the simplest one for you can be ...
- create AVAsset for both files (use AVURLAsset subclass),
- alloc AVMutableComposition (composition),
- add AVMutableCompositionTrack with type AVMediaTypeAudio to composition
[composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
- get track(s) from the first AVAsset and add it AVMutableCompositionTrack,
- get track(s) from the second AVAsset and append it to AVMutableCompositionTrack,
- then create AVAssetExportSession with your composition and export it.
Simplified description, but you get a clue. Depends on how many tracks do you have, what kind of effects do you want to use, etc.
If you do want to see some real code, open AVMovieExporter example, copy this code and remove video stuff and leave audio there only.
