When we say that Audacity is "free software", we mean three things:
- You don't have to pay for it.
- If you want to give it to others, you can do that too.
- You can even change the way it works.
The program is copyrighted, and is licensed to you under the terms of the GNU General Public License, version 2. The license is long and somewhat complex, but the short version is that there are only 3 major things it forbids:
- claiming someone else's work as your own,
- distributing a modified version of the program as a binary-only work, without including source code, and
- trying to restrict those to whom you give it from then giving it to others.
Because we say above that "you don't have to pay for it", that does not mean that you might not find people wanting you to pay for it, and that's allowed to: if they add enough value to justify what they're charging, the license allows this... as long as they, too, include the source code, and sa you might expect, this reduces markedly the number of people who might be trying to sell the program to you *solely* on the ground of modifications they've made.
As a rule, though, if you want to contribute to the program, and extend or modify it, you'll probably find that the best way to do that is to join the developer community, and submit patches, instead of "forking" the code, and going off on your own...