Why can't I hear the Halo: Reach firefight voices?

I have bought up to four firefight voices and the only time I can basically hear their voices its usually when my Spartan dies. I look on the Internet and youtube to see if anyone has had the same problem and discover not only do they NOT have this problem, but their fireflght voices throw out taunts and laughs (or something like that). Am I doing something wrong? Any answers would be appreciated.