This is a very delicate question.
You can always shout 'Hey!' But this is a little rude, and many people in addition to the one you are trying to communicate with will turn around and look at you.
You might shout, 'Hey,lady' or 'Hey, mister,' which would alert either the women or the men, but -- unless in an emergency -- this is a little rude, too.
I don't know that there actually is a way to communicate by voice. I think you might have to run up to that person, and say 'Excuse me, but you dropped your wallet,' or whatever the situation calls for.