a)There is not really a way to remove that background audio without affecting the main dialog. Adobe Audition/Soundbooth has a good filter that can reduce steady background hiss/humm/noise and that may reduce it somewhat. If possible the right way to fix this is re-record the audio if possible
b)When I have to edit audio that has some background sounds/ambient noise I usually grab a section of audio where there is no dialog and use that as a background replacement so when the main dialog drops out there is a some background noise still there and it doesn't sound so odd. Fadeing it in and out helps it seem natural too.
ChrisW
http://www.azprovideo.com
