I’m not a lawyer or expert but basically, GPL open source software HAS to remain open source (even if modified/updated). [u]Audacity License[/u]. It can be sold but anybody compile and distribute it (free or for a price).
There is a registered trademark so my understanding is that if somebody modifies it and makes their own version they can’t use the trademark/name. The modified version also has to be open-source.
The license terms place some requirements on anyone distributing modified versions. Primarily, the modified version must remain open source under the same, or a compatible license, and the modified source code must be freely available.
The main Audacity app is currently released under the terms of the GPL v2 license, though some specific files have separate (compatible) license terms (particularly some of the libraries used by Audacity). Most of the code is licensed under “GPL v2 or later”.