The loudness of the call doesn’t make any difference in terms of the speed of target release, the microphone will either pick it up or it won’t. If you’re having to raise your voice it’s probably because the microphone sensitivity has been turned down. Ground owners do this on windy days as the breeze across the microphones activates the trap.
When it comes to call, a couple of things may help. Firstly, make sure whatever call you use it’s a concise noise. A drawn out call that perhaps begins quietly and gets louder doesn’t promote consistency. Secondly, mount the gun with the mouth slightly open and use a call that avoids moving the lower jaw as much as possible. Something similar to an ‘urghh’ is best. Too often, when I’m coaching with the ShotKam I see muzzles dipping a noticeable amount when the target’s called for. This is usually down to the shooter opening and closing their mouth when they call for the target.
Ultimately, you’ll find your own call that ensures an instant target release and gun steadiness. Although some do call loudly, and shoot we’ll doing it, there’s no particularly advantage in doing so.