Why isn't the sound coming out of a movie i play on itunes?

I thought it had something to do with a conversion i made on some videos. But now i realized that any video i try playing on itunes are on mute, or something close to that. the videos play fine, yet no sound comes out of any video. so now i now its an itunes problem. I have a Mac incase thats an issue.
can someone please help with a solution?
Thanks!