Basically, I didn't want to spend money on wireless network chips and figured I could use the speaker+mic I was using for distance measurements for sending small bits of data too. It currently uses TDMA with 1 second time slots subdivided into 8 bits with sound being 1, and silence 0. The stunning 1 byte/s is good enough for me at this point. Embarassingly, they're still wired together to synchronize time. I'm mostly working on processing the sound to reduce interference (especially the motor noise). They can make a pretty reasonable guess at frequency, but it is getting hard to fit in the Teensy's memory. Maybe I'll get the new one.
As cool as it is to be sending data over sound, its only $25 for some of the cheaper wireless modules over at sparkfun:
http://www.sparkfun.com/commerce/categories.php?c=16
That would be impossibly more practical, and you wouldn't really have to have them wired together to sync them. You could have a master transmitter that sends out a sync bit that everyone syncs with (or something...).
For synchronization, you could use a pattern of start/stop bits as used by RS232. It would take a few bytes to get synchronized, but it would probably work.
