Noiz2
Sound Ninja
What you are looking for is made by Dan Dugan. It's an auto-mixer. It was more designed to deal with a situation where you have a panel of folks all miced and want to shut down the unused ones but it will do what you want. However it is $$$$.
However that is generally a bad way to deal with it. A good mixer and a proper post will do much better for you.
However that is generally a bad way to deal with it. A good mixer and a proper post will do much better for you.