What sort of assets does PHAssetMediaTypeAudio fetch?

Question

I am testing PHAsset more particularly the following method:

+ (PHFetchResult *)fetchAssetsWithMediaType:(PHAssetMediaType)mediaType
                                options:(PHFetchOptions *)options

I am unclear what PHAssetMediaTypeAudio and fetchAssetsWithMediaType:PHAssetMediaTypeUnknown actually retrieve. This is my test code for illustration:

- (void)testRetrieveAssetsFromPhotoLibrary {

   // this gets me photo library images
   PHFetchResult *imagesResults = 
     [PHAsset fetchAssetsWithMediaType:PHAssetMediaTypeImage options:nil];
   NSLog(@"Number of images: %i", (int)imagesResults.count);

   // this gets me photo library videos
   PHFetchResult *videoResults = 
     [PHAsset fetchAssetsWithMediaType:PHAssetMediaTypeVideo options:nil];
   NSLog(@"Number of video files: %i", (int)videoResults.count);

   // what does this get me??  
   // not iTunes sync'ed music nor Voice Memo recordings...
   // prints 0...
   PHFetchResult *audioResults = 
     [PHAsset fetchAssetsWithMediaType:PHAssetMediaTypeAudio options:nil];
   NSLog(@"Number of audio files: %i", (int)audioResults.count);

   // what does this get me too??
   // prints 0...
   PHFetchResult *otherResults = 
     [PHAsset fetchAssetsWithMediaType:PHAssetMediaTypeUnknown options:nil];
   NSLog(@"Number of other files: %i", (int)otherResults.count);

}