Hey, I'm Ayden. I did all the music and music systems for this game. If you're wanting to make music/sound an important part of your game, I'd recommend getting some middleware software to help handle the audio, either FMOD or Wwise. We used FMOD; all the music runs through one FMOD Event. In the Unity Engine, a script reads the beat information from FMOD, and from there we can lead events to happen every time the beat changes, including triggering animations: https://qa.fmod.com/t/sync-game-events-to-music-beats/12111/2
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I composed all the music myself in Ableton Live. If you're not familiar with writing music, then you'll want to be looking for royalty free music online. Here's one possible source: https://www.dl-sounds.com/royalty-free/category/game-film/video-game/ If you can't find what you're looking for here, there are countless other royalty free music sites elsewhere online, so it should just be a quick google search away.