I guess I should have looked more into what a Wilhelm Scream is. As I was posting my question, I was in the middle of researching public domain footage for a project I was working on. I saw your post and just plugged in "Wilhelm Scream" into the search for kicks to see what came up, and that was the link.
Everything on that site is public domain, including footage, audio files, and much more. If it's on that site, it's public domain. My mistake for not doing a bit more research into what a WIlhelm Scream is, sorry :)
Be careful of that website. I've looked for footage there and someone marked it as no copyrights and trust me, it was copyrighted. The work I looked at was a bunch of copyrighted footage strung together in a montage. Maybe the person who submitted it was saying that the work they did in stringing the footage together was not copyrighted but the footage was and that was not made clear. Maybe it was a one time thing but it bugged me. Thanks for thinking about me and offering up help, I appreciate that.