As one of the resident sound people I can tell you that $600 will not even cover a basic entry level sound kit. Now, you can definitely do things to improve your audio on a much smaller budget, but then you have to pay meticulous attention to capturing solid production sound - we'll leave audio post for later.
Two of my favorite aphorisms are "Your project only looks as good as it sounds" and "Sound is half of the experience." Even though I am a "soundie" if you ask the more experienced shooters here on IndieTalk and on other forums you will find it to be true; audiences are quite forgiving of marginal visuals as long as the audio is solid.
Filmmaking is the integration of two technical disciplines - visuals and sound - and you need to pay equal attention to both (we'll leave the creative aspects aside, for now). In fact, the smaller the budget the more attention you will have to allocate towards sound. That doesn't mean spending gobs of money, but rather making sound an important priority in your thinking from script to post.
I know that you probably won't do that, but you'll learn...