I have an MKV file with H.264 video and AAC audio. With FFmpeg the default audio codec for MKV is AC3, so if you run a command such as
ffmpeg -i infile.mkv -ac 2 outfile.mkv
it will transcode the audio to AC3. I can run this command and all is well
ffmpeg -i infile.mkv -ac 2 -c:a libfdk_aac outfile.mkv
However is a way available to say "use the input codec"? Note that
copy will not work because I am making a change to the audio stream.