Yeah, having frequency at just 6000 will make the filter remove a lot of frequencies that should be kept. Try raising frequency to 12000 or higher. Actually, as high as you can go before the sound returns.
FIR is short for Finite Impulse Response, but that's really just the math behind it. I guess it could have been named something better, but well, it's not, and I'm not sure what that should be either
A mod: This thread probably belongs in tech support.