Firstly, the most importent part to remain aware of this problem while you speak.
and for the active part: you can't really scream too hard while playing the ball, so just have at it - scream to the guys (just don't high pitch, heh) as hard as you can for start (if they give you a hard time, say you are a bit drunk

), and with time you'll learn to ajust the tone.
if you really can't shout, do some breathing exercises - stand with your back streched and do some breathing from the belly (make sure your chest doesn't move, but only your stomach).
don't be afraid to overdo it, it's the way to learn.
btw, the problem with strangers is a good thing, it gives you the possibility to train on non-targets..
last thing - drop a quick word here if this method works or not, I'm working on tonality myself