This is how we can extract the audio from Video :

First import the video from libaray using imagePickerController delegates methods. 

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
  // 1 - Get media type
  NSString *mediaType = [info objectForKey: UIImagePickerControllerMediaType];

  // 2 - Dismiss image picker
  [self dismissViewControllerAnimated:YES completion:nil];

  // 3 - Handle video selection
  if (CFStringCompare ((__bridge_retained CFStringRef) mediaType, kUTTypeMovie, 0) == kCFCompareEqualTo) {
    self.videoAsset = [AVAsset assetWithURL:[info objectForKey:UIImagePickerControllerMediaURL]];
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Asset Loaded" message:@"Video Asset Loaded" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
    [alert show];
  }
}

then we can extract the audio track from Video file using following method:

-(void)extractAudiofromVideo
{
   
    AVMutableComposition*   newAudioAsset = [AVMutableComposition composition];
    
    AVMutableCompositionTrack*  dstCompositionTrack;
    dstCompositionTrack = [newAudioAsset addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    
  [dstCompositionTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, self.videoAsset.duration)
                        ofTrack:[[self.videoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];
    
    AVAssetExportSession*   exportSesh = [[AVAssetExportSession alloc] initWithAsset:newAudioAsset presetName:AVAssetExportPresetPassthrough];
    
    exportSesh.outputFileType = AVFileTypeCoreAudioFormat;
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *audioPath = [documentsDirectory stringByAppendingPathComponent:@"soundOneNew.caf"];
    exportSesh.outputURL=[NSURL fileURLWithPath:audioPath];
    
    
    [exportSesh exportAsynchronouslyWithCompletionHandler:^{
        AVAssetExportSessionStatus  status = exportSesh.status;
        NSLog(@"exportAsynchronouslyWithCompletionHandler: %i\n", status);
        
        if(AVAssetExportSessionStatusFailed == status) {
            NSLog(@"FAILURE: %@\n", exportSesh.error);
        } else if(AVAssetExportSessionStatusCompleted == status) {
            NSLog(@"SUCCESS!\n");
            
            
            NSError *error;
            
        }
    }];
}