There is an outstanding free software program that I've used repeatedly for a variety of audio-video conversions that will let you strip audio from video and video from audio. It is at
SUPER © .. I've had it on my laptop for over a year now and (knock wood) have had no problems with it. You just drag your file into a box, check the options in the window that you want to see happen, and click "encode." I just used it a minute ago to strip and save the sound from a free french lesson onto my cell phone chip so I can practice it anywhere.