Here in the blogosphere, we only care about up-to-the-minute technology, right?
State-of-the-art 1995 technology, so get cracking: a piezo sensor
mounted to the front face of the balloon lets the ballon's aluminized
mylar body act as both microphone and speaker. In layman's terms: the
balloons can talk to each other. (Don't say "I invented talking
baloons" and expect to get far in academia, though. The proper term, as
in creator Joseph Paradiso's article for the IBM Systems Journal —
fine bathroom reading, by the way — is "The Interactive Balloon;
Sensing, Actuation and Behavior in a Common Object.")
Yes, that's right, they don't talk to each other, they actuate each
other. Now go, read the paper, and build yourself some baloon speakers
for your next gig.