For some reason I've been fixating on sound effects for Project Alexandria. I've been working on a sound for the aliens to use when they get hit. I just wanted to drop some notes that I've found useful:
If you're aiming for an NES sound, the decimator is your friend.
You can also use the "Random Wave Generator" LADSPA plugin. In Audacity, you can use the FCSA variant on some random audio track, and specify a low-ish frequency to get a lo-fi white noise. I think this is how I made the "big explosion" which you hear at the end of training5.
I'm currently working on doing some kind of frequency modulation from within audacity, so I can make a sound that starts at a high pitch and drops a little. Will this sound good, or at least better? I have no idea. Audio stuff is hard!
[Edit: It's called "chirp", it's in "generate". Sigh..]