OpenGameArt.org
the inner world mp3
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"the inner world mp3" by Deeppixel licensed CC0: https://opengameart.org/content/the-inner-world-mp3 Author's attribution notice: It is free, you can do what you want with it. Enjoy :)
Where to put it: Game credits or video description. If the entry has a "Copyright/Attribution Notice", follow that.