It looks like it requires a MIDI which it then converts to sound like it’s coming out of a Gameboy.

Here’s what the FAQ says:

> How it works

>

> Search a song, pick a MIDI source, hit Generate. The Wario Synthesis Engine analyses the MIDI and resynthesises it using Web Audio oscillators tuned to mimic the Game Boy's 4-channel sound chip. All processing runs in your browser.