ギター Hero: メタリカ - Impossible Vocals?

Just wondering if anyone has gotten through the career on vocals?. My voice is naturally very deep and I think it screws me over in this game. I have no problem changing the pitch of my voice but the game is entirely unresponsive to it. My kid brother is 12 yrs old, and has a voice like a little girl and so far he's managed on medium difficulty - I however can barely get through anything on easy. I'm also using my xbox headset as my mic so I'm wondering if the unresponsiveness could be attributed to that, and not having a real mic. Ive tried moving my mic, farther/closer to no result...any closer and i'd be eating it.