Yes I've seen those, but the problem is they use the same technique I use with the theremin, i.e. something like:
to generate sound, which is ok, but is quite restricted.
But the article I linked to, which is the next one in the series - Arduino Interrupts - explains how to generate sound with a more advanced technique, which is using the built in timers/counters. And they say they'll write new articles on sound using this, so we'll just hope it's coming soon!