I have several videos that have video, AAC audio, some with an additional alternate content audio stream, and subtitles.
What I want to do is to copy EVERYTHING as-is while inserting a converted a:0 from AAC to AC3 into the a:0 position (so all existing audio streams are "bumped" down by one spot) and making the new a:0 the default audio stream. End result is the same video file but with one new additional audio stream. (AAC stream is kept, not replaced.)
I've tried this, but I think I'm doing something wrong because it seems to take a long time (compared to converting the audio and then merging it in separately).
ffmpeg -i "AAC.mkv" -map 0:v -c:v copy -c:a:0 ac3 -b:a:0 640k -map 0:a:0 -map 0:a -map 0:s? -c:s copy "AC3.mkv"
It does appear to work, but as I said, seems to take longer than it really should. So, am I just being impatient, or am I doing something wrong that is slowing it down?