I'm not sure you should worry about making commentary files right about now. Quite a few people are working on them. I've done about 20, I've sent them in to BLC and hopefully they'll be on the site in a while.
If you're still interested you can get a program named Goldwave off the net.
I used the Goldwave and Sound Rec. and essentially you're just cutting out the parts you don't want from a particular commentary file and pasting them together.
You need the Goldwave, if atleast just to save them. As the saved files in the sound rec. won't work in the game.
Good Luck
