IÂ´ve done a bit of this...also using Audacity.
As Oly says, you can sometimes reduce general background buzz, but what can be most annoying is the occasional inappropriate word or comment. How to deal with that is simple but requires care.
Essentially, you cut that section out...sometimes itÂ´s just a second...
Now...the part where you have to be careful: select Ã¡ similar "normal" background buzz/chatter and copy and paste it into where you cut the bit out. You can add audio blending effects if necessary. No-one will notice! IÂ´ve done video with sections several minutes long where the audio came from a completely separate part. In one example, someone started talking to me without noticing the camera was running (I was filming the police while someone was being "interviewed" after a disturbance). I had to blur parts of the interview anyway, but I was able to more or less entirely remove what the other person said, without anyone picking it up.