There's a simple way to measure the speed of sound. It's low tech and unimpressive, but you get to blast an air horn. How many times can you say that you're using one for science and not sheer obnoxiousness?
Sometimes the simplest solution is the best solution. That's not the case here, since measuring the speed of sound precisely depends on some sophisticate equipment. Not only does the measurement have to be exact, but other factors, such as temperature and humidity come into play. For those not interested in delicate engineering, though, making a general estimate of the speed of sound is a straightforward process.
Find an area with plenty of flat open space and no one around to call the police and make a noise complaint. Armed with a buddy, an air horn (or anything that will make a loud noise on cue), a spool of string, and a timer or watch, stake out a place and get started. Your friend will hold on to one end of the spool of string and start blasting the horn, clanging the cymbals, banging the drum slowly, or just making whatever racket you two have agreed on. You will back up slowly, holding the other end of the string and using the timer to measure the time between when you see your friend making the noise and when the noise hits you.
At first the sound will seem to reach you at pretty much the same time as the light does. However, as you back farther and farther away, a delay will become apparent. Have your friend keep making the noise until you get a few consistent measurements for the time delay. Note them down in seconds, and mark how far along the string you are. By measuring the length of the string – and so the length of the distance between you – and dividing that by the number of seconds you measured as a delay, you'll get the approximate speed of sound.
This seems like an easy explanation, but it also comes with a challenge. The speed of sound in air is about 343 meters per second. The speed of sound in sea water, where you can also do this experiment, providing you have a big body of water, waterproof equipment, and are making sure you aren't deafening whales, is 1500 meters per second. But what is the speed of sound in gummy bears? In marshmallow? In chocolate sauce?
Io9ers, go forth and measure. If we don't have a calculated value for the speed of sound in a melty ice cream sundae by the end of the week, I will consider this entry a failure.
(If you're not willing to experiment, at least write in and tell me how many of you want a sundae right now.)