moddb is a good place to find one. However, you might want to hit the page below to learn a bit more about the modding process in general:
Mod (computer gaming) - Wikipedia, the free encyclopedia
In short, there is an "editing software" for it, but it varies greatly depending on which mod you work on. Each mod is based upon another game, which has its own engine. So it might be a mod of Half Life, or it might be a mod of Doom, or something to that effect. There are several engines you will potentially encounter (Source for Half Life, Doom Tech for Doom, Unreal for Unreal Tournament and Gears of War), and each will have its own process for creating and implementing sounds.
A big part of this equation will be having a PC powerful enough to handle this. And before you dive in head first, it might be wise to get your hands on a moddable game that you think you'd like to work with and tinker with its sound tools, just to see how things work. You should find plenty of documentation available on the web to help guide you as you get your feet wet with this, you just have to start searching for it.
Best of luck!