However it only took the first audio stream regardless of language. I had a ffmpeg command that would successfully remux an mkv to a iTunes compatible mp4.Due to the efficient ratio between video quality and file size and streaming opportunities, MP4 is commonly used to distribute video via the Internet. MP4 supports compression, subtitles, multiple data types, and streaming. The main video codecs used in MP4 visuals are MPEG-4 and H.264.