you will have to play with that i am not sure. but i am pretty sure in the timeline you can hit the plus button and that will show you the video audio seperate from the video. then you can start stop or turn it down when ever you want throught the movie.
hope that helps. there is a learning curve your movies keep getting better the more you use it.. so just start messing around. and dont forget to post them all on here so we can laugh at you. lol