Those are all frequently asked questions. They are also guaranteed ways to kill your show. You quality for 1 and 4.
-- The Four Horsemen of Audio Recording (reliable, time-tested ways to kill your show)
-- 1. Echoes and room reverberation (Don't record the show in your mom's kitchen.)
-- 2. Overload and Clipping (Sound that's too loud is permanently trashed.)
-- 3. Compression Damage (Never do production in MP3.)
-- 4. Background Sound (Don't leave the TV on in the next room.)
We would have no trouble removing one simple, perfect echo, but they're not simple and they're never perfect. Echoes are the speaker's voice arriving at the microphone more than once offset by time. So right away, we're telling to software to remove the performer from himself. Everything in the room affects the quality of the echo and makes it less and less likely that anybody's going to be able to help.
Oddly, this same problem occurs at the other end of the chain. "Put a good echo into this voice." Those always sound fake because real echoes are insanely complex and you can't generate insanity with normal computers.
Not easy this echo thing.
The other question, the lower than a certain level thing is doable. There's a tool for that called a gate that only opens when the sound goes over a certain volume.viewtopic.php?f=39&t=30996