Extracting the Audio
 

Is there a cheap standalone program that can extra audio from a DV AVI file?

Specifically the L/R channels into two mono-channel files?

Can standard sound editor do it?

What is SoundForge I keep reading about? Do you have to pay for it?

BTW, can any of these sound editor remove hissing sound from pumping up the volume too much?

"Rad Video Tools" (free/shareware) will extra audio from a DV AVI file, as would Quicktime Pro (also very handy to have but not free).
A "demuxing" program will seperate audio from video for things like MPEG2 streams from DVDs. You'd use that if Rad/QTPro won't work.

Any sound editing program should be able to seperate a stereo file (generated by the above programs) into any combination of L/R mono/stereo files. Most NLEs will do it too. Pretty basic stuff.

SoundForge is a sound editing program which doesn't do multitracking. It's good at removing hiss, background noise, etc. You have to pay for it (not that much if you qualify for academic discounts). Keep in mind that fixing up sound in post is a very bad idea. You'll spend lots of time fixing sound and still not get it perfect. Getting sound right during a shoot is very important!