ffmpeg c program video to mp3 - open to bidding

Using c++ and ffmpeg convert a video file to mp3. Not using command line args, but using ffmpeg libs like libavutil, libacvodec etc Basic idea is get a input video file identify the audio stream encode it to mp3 and write to a file. If it is a MP4 input the audio stream will be in AAC , so transcode AAC to mp3 and write. Only this portion of code is required