I just noticed while prototyping that using a DC wall-wart causes audio artifacts if I amplify the sound too loudly. I'm assuming this is due to noise (batteries are quite clean power sources)-I thought I had a decoupling capacitor on my voltage rail for a 7805 circuit the wall wart is also powering, but it turns out that the capacitor is on the output side of the regulator, not the input. D'oh!