u can aways use imported sounds as mp3 24bit 24,000khz which works pretty well and is tiny.
of course you need an animation trigger to attach the sound to the unit when it attacks and make the sound 3d with and set the distce to something like 0 1500 1600 or whaterver u need.
alternatively u can use a 22,000hz mono wav replace the internal sound set used for the attack sound.
Now pick your sound :)
http://www.grsites.com/sounds/battle001.shtml