One reminder to anyone intending to play with new sounds: Make sure you normalize the volume with the rest of the in-game sounds, to make sure it's not too quiet or too loud by comparison. They'll really stand out otherwise.
While there are programs that can do this, I've always just "played it by ear", setting my headset and player volume to a specific level whenever editing sound effects, and comparing sounds to the generic rifle sound (RIFLE.ogg) which I have sitting on my desktop for that purpose. That and looking at the waveform in Audacity to keep certain categories of sound within a certain range.
Just some advice for getting the best results that will fit better into the existing assets. Also note that if you use anything from freesounds.org, check the license since you're usually supposed to credit the author in the readme.
Yeah, the quality and content on freesounds is pretty random.
Audacity is nice, but probably takes some time to get used to if you're not familiar with sound editing in general. It does lend itself to just playing around with, though, and there are plenty of tutorials online.