How do i sperate Audio from a video so i can turn it into a WMA file?

i wanna make a quick video using Window Movie Maker and im having a little issue. I have a video (even tho its auto its a video file) and i wanna turn it into a audio file so i can put it in a video so it plays with the rest of the video. Also i have a video i downloaded with Real time player and i wanna put that into a new project using Windows Movie Maker but can't... anyone able to help? <3