I just got finished installing an aftermarket Cruise Control on my new Honda Insight, and after a bunch of reading on the 'net, I can tell you there are two main ways people use to gather speed info from a car:

1) Alternator noise, as listed above... note that this doesn't work with my car since it's a hybrid so it's electrical system is.... nonstandard, to say the least.

2) tap into the spedometer cable. These days most spedo's are apparently electronic and work in some number of pulses (voltage spikes) per mile... I had to set my cruisecontrol to 4000 pulses per mile, with 8000 apparently being another common setting. This would seem to me to be the easiest way to add speed-based volume control to your system: tie the pulsing voltage of your spedo cable to some input on the serial port (CD maybe?) .

Just a pair of pennies that I hope you find helpful...

--pj