The people in the background are not noise. They're performers. In general, you can't separate different performers in a show. Noise is one constant sound through a whole performance like air conditioner rumble or microphone hiss. There is another requirement that at some time, the noise has to be available by itself alone so the software can tell what it is. This is the "profile" step.
Noise removal works in two steps. Tell the software what the problem is -- the profile step where you play the noise by itself -- and then the software tries to remove the junk from the show in step 2.
If the interference changes or you can never get the bad sound by itself for a bit, the noise removal tools will not work.
If you do want to mess with it anyway, only the noise removal in Audacity 1.3 actually works really well.