The game hitsounds system centers the pitch at 50 damage.
That means if you hit with raven/violator (~20 damage) the pitch will be around 2, and if you hit plain rocket or grenade (100damage) the pitch will be around 0.5. Unfortunately you cannot change that (yet?).
You can preview in UDK how it will sound, by changing the PitchMultiplier value in the Sound cue properties. However, you cannot change the way it is going to sound ingame (because, as I said, the game will override that value with the damage formula).
If you really need to re-center the pitch, you will have to edit the sound at its source using an audio editor.
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