Audio Transcoding on iOS

I grab the song data from the iOS music library (as an object NSData), but when the data is not related to the MP3 file, I would like to transcode it.

I have two questions about the transcoding process:

• Is there something that could be done, say, M4A to transcode MP3s, or will I need to use something like ffmpeg or other external libraries? If the latter, are there any examples of this on iOS knocking about?

• What is the best way to do this so as not to load everything into memory? Obviously, a large song file will run out of memory, and the application will be terminated.