I think if the OP wants to handle the legal ramifications properly on his project then he can't depend on a YouTube video upload. Furthermore, you can't really trust a random website with downloadable sounds to be on the "up and up".
There are online services that, for a fee, provide sound effects in which the copyrights are controlled by the service. Many of these are of the "royalty free" variety...if you've paid the upfront fee then you owe no royalties for subsequent use. Some may require a credit in the film.