I have two folders full of movies in mp4 container. One folder has 7.1 AAC track first, AAC stereo track 2nd. The other folder has a DTS track 1st, AAC stereo track 2nd. For compatibility reasons I would like to convert that first audio track in both folders to 5.1 AC3 and leave the stereo intact without changing the video stream. Can someone help me with what commands to use? I am new to ffmpeg, and the best I have been able to do is adding a 3rd audio track. If possible I would like to end up with just the 2 tracks. Thanks.