In my search for some more awesome music-related Arduino things, I found Miditones and Playtune. Miditones takes a MIDI file as input and outputs a C array that you can copy/paste into your Arduino code and play it using the Playtune library. I just used the example code for an Arduino Nano, with pins 10, 11, and 12 going to three different speakers.
I didn’t realize until just now that the comment in the code says that you can wire all three pins to a terminal of one speaker, but I guess having three speakers makes it kinda surround-sound-ish. Maybe I’ll upload some more videos with a single-speaker setup or with a piezo buzzer. Anyway, here are some videos of Vocaloid songs being played with the Arduino.
Edit: Here’s one with just one speaker.
Edit2: La Campanella and Happy Synthesizer.