Hey - I just successfully did this with some lightsaber sounds.
First, in the object editor for the TARGET of your attack;
Take note of its Combat - Armor type (this is not actually anything to do with armor type. It just determines what sound is played when it is hit)
Now, for your rileman (or other attacker) find its Combat - Attack 1 - Weapon Sound
Then open up sound editor, go Ctrl + F and type in your ATTACKER'S Weapon Sound followed by your DEFENDER'S armor type. It will look something like "MetalLightSliceFlesh". There will be ~3 varations of each type.
Right click on each of the existing sounds and go 'Replace internal sound' - then point to the sound you want to use
Since the game takes into account both the attacker's weapon and the defender's armor, you may have trouble when it comes to attacking other units
Hope this helps!