Well, I always thought he had a voice changer in his mask, in order to make himself sound scarier. Remember, he sounded that way to Rachel at the end of BB, who was immune to his gas at the time. The whole scene is just lazily performed by Murphy, which is sad because I really like him as an actor - those two changes would fix his performance 100%.
Another cheesy line my friends reminded me of: "No more dead cops!"