I have to use the following command for ffmpeg
ffmpeg -i \"Video.mp4\" -map 0:1 \"AudioWithVocalfromStreamOne.mp3\" -map 0:2 \"AudioWithoutVocalfromStreamTwo
Best method of using FFMpeg for IOS is the MobileFFMpeg Library.
Here is the source: https://github.com/tanersener/mobile-ffmpeg
It is available as a pod pod 'mobile-ffmpeg-full', '~> 4.3'
Make sure to get appropriate FFMpeg license. Different licenses have different libs
License to choose from: min min-gpl https https-gpl audio video full full-gpl