Heya
Help is of course always appreciated!
The sound library prospector uses can read .wav, .ogg and .mp3
There shouldn't be a size limit, though of course blowing up the zip file to 500 GB would propably be a little too much

As to what sounds would be nice: I have to confess I never thought a lot about it. when I play I always use the nosound version.
hmmm....
Just brainstorming here:
A computer voice for the warnings was of course always something I would have preferred to the beeps.
Sirens for entering space combat?
we now have lasers projectiles and missiles for space combat, but I don't the sounds reflect that
Atmospheric sounds for planets/docked derelicts?
Breathing sound for planets without atmosphere, animal noises for planets with animals, murmuring and crowds for when you get close to other people? blowing wind, crackling rocks etc?
erm... cave sounds?
Robots?
waves?
As I said, just brainstorming