itch.io

Extra? You never payed an upfront cost to begin with. It's not rocket science to take one look at the OC's page and see that they have purchasable items on their Itch page, which probably means, no, they would not be happy with you taking their kindly made Royalty Free work and feeding it into AI where it will then become apart of a larger ecosystem of AI trained sound effects all based on their hard work. There's also a license, if you've even read that!