The amount of sound required to fire the trigger depends on the trigger you get
The Hiviz sound trigger is cheap, but kinda crappy. It uses a piezo buzzer as a microphone, and isn't very sensitive. Snapping fingers, clapping hands, and firing a gun will set it off, but talking loudly or yelling won't. So popping a water balloon usually will not set it off.
Also, there's no latch to the circuit, so multiple loud sounds could trigger the flash more than once. For instance, a friend and I were firing paintballs at a piece of plywood, trying to capture the impact. Several times the trigger fired on the sound of the gun firing and the impact of the paintball on the plywood (only microseconds apart).
I'm an engineer by trade, so I'm working to tweak the sound trigger and make it work a little better. First off, I'm getting rid of the piezo buzzer and replacing it with a more sensitive microphone...which requires you to change out a couple of the resistors in the circuit. The second part (and most difficult), is trying to integrate a digital display into the circuit...I'm still working on that, but hope to have something built in the next few weeks. When I have a good design, I'll probably post the design on my blog.