BARF!
I don't get how people can't just be happy with themselves... that aside.
I get what you mean by the hollow sound.
Here is some theory on sound for ya.
I am going to assume you are still using the imac mic.
this MAY or MAY NOT work.
place cardboard on the left, right and if you can, the top of your imac to create a box. Then try and use your mic. It might be a little more boomier, but you can reduce the bass if you like. This is what I do when I record guitars, except I use plywood. But in my ghetto days of recording, it did the trick.
if not, take a chance with a cheap, USB "skype" headset off of ebay. It's about the proximity to your mouth the mic is. You just have to turn the mic a little off axis to your mouth so that you don't get wind pushing into your mic.
Savy?