How can I separate the audio from an existing video file on iPhone programatically?
iMovie for iPhone 4 does not offer the ability to separate the audio from the video of a movie. It only allows one track of audio - either a background song or the audio from the clip you're using. If you click and hold on a clip it brings up the Clip Settings view where you can turn off the audio for the clip.
iMovie for iPhone 4 doesn't allow you to do anything programatically. If you're wanting to develop something with the iOS SDK to pull the audio from a movie I suggest editing your question and reading up on what you can do with AVFoundation