audacity how to remove echo

Echoes are one of the killers. An echo is the actor's voice arriving at the microphone twice, once direct and once late having been bounced from a wall. So you are asking the software to remove the actor's voice from itself.

The expander (see above) and level keying software all talk a good line, but none of them work very well because of other nasty things they do to the show.

There are ways of doing this, but they're completely impractical. Somebody stands up in front of the audience with a sound control generator and creates a sound profile for the computers to later figure out where the walls were and their characteristics.

It needs to go on right after the warm-up band, but before Nirvana.