Gamespy has a great article of the music from Spore. Audio engineers Kent Jolly and Aaron McLeran speaked to a crowd at the Game Developer Conference on how they brought their dynamic tunes to life.
The team used a special version of a software tool called Pure Data (or Pd), customized by Electronic Arts and affectionately called EAPd. Pd has a graphical interface that visually shows you events that can be linked together with lines, creating a kind of flowchart to describe how sounds are created. Jolly gave a quick demonstration of how to create a nifty conga drum loop: Gather together some drum samples, use a metronome routine to trigger off a random drum every beat, and loop it every eight beats… presto! Instant drum loop. This was achieved with only a few boxes in Pd.
Thanks to Spores Illustrated for bringing it to attention.