As a musical sort of guy who now dabbles in animation, I have often made use of Freesound, a collaborative database of Creative Commons Licensed sounds.
I often need Foley sounds in my musical works and now animations. Sometimes I create them myself but I also make use of this fantastic resource from time to time. Sound is an important component of animated storytelling so having easy access to sounds when creating your animations is a great time saver.
If you only use CC0 rated sounds then no attribution is required. The different licence types are listed below.
zero (cc0): http://creativecommons.org/publicdomain/zero/1.0/ attribution (by): http://creativecommons.org/licenses/by/4.0/ attribution noncommercial (by-nc): http://creativecommons.org/licenses/by-nc/4.0/
I tend to use cc0 nowadays as it just makes it easier when creating.
I thought I’d post a link to the website here as I feel it is an asset, just not a visual one.