It's worth adding that a charming feature of musical gyroids is that they run in sync, on short loops. With, say , 8 gyroids, you have a lot of scope for composition. Set off a bassline, pitch in some bells, select some percussion... they all loop independently, so the point at which you set one off is an important factor. There are also some gyroids that make noises at random intervals, and adding them gives your loop some necessary variation. Gyroids also seem to keep time with the various songs you can play on your hi-fi equipment.
The 15-voice limit is rather annoying (especially as the GameCube has 64 audio channels), since it's difficult to arrange a large collection of gyroids so that you can turn them all on or off as you like (you need to be standing next to one to do this). Quite often, I find a gyroid has been automatically deactivated by the voice limit, and is impossible to reach without a major furniture reshuffle. I suppose the voice limit allows other ambient sounds (the ticking of a clock or clacking of a Newton's Cradle, for example) to play unhindered, as well as any music you might put on.
I haven't yet seen any gyroid compositions posted on the web, but if they aren't there already it can only be a matter of time. A possile follow-on project for Nintendo Teenage Robots, perhaps.