Things like monster sounds are easy - you could pick up a cheap mic for £5 and get some cool sounds for your zombies etc. Explosions can be made as well, by blowing on the mic and slowing the sound down and playing with filters - there's plenty free sound editors available.
Adding your own sound effects to FPSC could'nt be easier, it'll usually just be a case of selecting the appropriate wave file when an action is carried out. Besides graph paper, it the other thing you can be looking into while waiting on release.
Van-B
It's c**p being the only coder in the village.