You probably won’t be able to do what you’re attempting with the Dev Kit. The output to the speaker can only be made high or low, under software control. It’s likely that more complex sounds could be created using high speed pulse width modulation (PWM). There has been discussion about moving the speaker to a PWM capable pin on the production version of the Arduboy.
@ChrisS recently posted about playing externally generated complex sounds:
Yeah with PWM pin as planned will get us a bit more options. The piezo also limits sound quality so even with proper drive hardware it’s a challenge to get anything to sound right other than basic 8 bit noises.