In your head?
I'm serious. What sort of sounds do they make, in your head, when you are playing them?
I play a variety of armies, so my ideas are quite varied. I picture Imperial weapons as having that high-pitched firearms "crack!" and lower artillery "boom!" depending on the weapon. Except for lascannons, which are a sharp high-pitched hiss, and melta weapons, which make more of muffled "whump" noise (followed by sizzling, of course). Plasma weapons have that traditional sci-fi "pew pew" noise, getting larger as the weapon increases in size.
Eldar weapons, on the other hand, are all eerily silent. Their shuriken weapons all make only the faintest hiss as the monomolecular edge weapon cuts through the air. Even their beam weapons only make the nearly inaudible sound of the air being ionized and displaced by the laser or lance shot. Of course, the results are noisier - the thud of shurikens into flesh, the scream of tortured metal - but the shots themselves are nearly silent.
The Tau are somewhere in-between. Their mechanical weapons - missile launchers and such - make audible clicking noises as new rounds are chambered, and the missiles have fairly noisy rockets and fairly noisy explosions. Railguns are accompanied by increasingly impressive booms. The various plasma weapons make quiet, muted "pew pew" sounds, while ion weapons just make a staticky hiss.
What about you? What do you imagine while you're playing?