Here is an internet
collaboration system
from auracle.org
for voice activated sound performance. It's
plug and play, but it might want to install
the java softsynth software on your computer.
If you find yourself all alone in Auracle's world, call up some friends and have them plug in -- or just turn your speakers up (a little!), and create a loop between your speakers and your microphone. Careful, feedback loops can gain amplitude quickly. You'll have your whole audio library to experiment with then.
The java softsynth website has many other audio toys to play with on this software.