Audio (music or sound effects) with a GPL-or-free’er style license, such as CC BY-SA, CC BY, and CC0.
Note that this does not include audio that is provided as "royalty free" or "use however you want" without specific license terms. Copyright law (and immaterial rights law in general) is a complicated mess and without a proper license guaranteeing certain freedoms, the assets can’t be considered truly free (as in speech)/libre.
If you’re less concerned about your freedom to use the audio in different ways, feel free to check out my other collection Gratis music which contains music that is free to download but do not have a sufficiently permissive license explicitly declared for it.
CC BY
