free post production software? I'd recommend the iTools (GarageBand and iMovie) for audio and video editing. Audacity is pretty good, and it's free, but it just works with audio.
I'm not sure you realize how important that background audio is. It may be barely noticeable, but like I said, you notice when it's too quiet. The reason we record room tone is two fold;
1) it can be used to establish a noise signature that can be used to remove some noise
2) if you need to cut something out of your audio track, and you don't have that room tone to put in it's place, you'll have a dead spot that will be very noticeable.
In your case, you want ambience to make the scene seem real. The only time it's dead quiet in the real world is at 4AM after a fresh foot of snow has fallen, and just before you're going to be pummeled by a tornado. Haven't you ever heard someone say, "It's too quiet". Too quiet is ominous ... unnatural.
However, you want to record your ambience separately, so you can mix it in at a very low level to keep it from detracting from your dialog.
Hopefully this all makes sense, but if it doesn't, just go out and record your movie. If it doesn't work the way you expect, you'll learn a whole bunch from your mistakes. Then you go out and do it all again.
So, regarding your question, did you mean to ask about a free Non-Linear Editor for OS X ? Post production covers a very wide range of tools.
Doug