I'm working on an Arduboy clone. The original schematics show that a speaker is attached between pins 5 and 13:
...along with a 4.7 µF capacitor.
The Arduboy lib has two sound objects, one operating on pin 5 and the other on pin 13. Does that really work? One can send a simple square wave on either and the single buzzer beeps that tone. And if both pins send their own frequency, some kind of mean wave will operate on the piezo element, creating a sound with both frequencies audible?