I'm very much aware of what public domain means... after all the concept is enshrined in the American Constitution. Your point appears that any utterance is public domain. I'll go for that in a very simplistic form since it is uttered once, is in final form, and cannot be copied--but the knowledge of what was uttered has been shared. Public knowledge (property) should not be confused with Public Domain, which defines when the creator's right to copy ends by law (and thus can become public property). If not in public domain, it is copyrighted.
Video files are not an utterance. They are tangible and can be duplicated/distributed, and may contain written works, art, music, and sound effects which may have additional copyrights. Is putting a vid on youtube or viacom a tacit declaration of Public Domain (ie. waiver of copyright)? I don't buy that concept. YMMV