In live field recording of random samples of otherwise UNcopyrighted content, i.e. people conversing or making formal announcements, babytalk, animal sounds, 1) is it necessary to obtain signed releases from any and all humans involved? or is it treated like candid photographs which have a limited usage without permission.

Interesting subject. Be aware that the advice that you receive here is worth about what you paid for it. I am not a lawyer but I'll give you what I think about the subject.
If the sound samples contain no direct implications to any company (as in airline gate announcements etc.)or in any other way identify the speaker, then I think you are safe to use them. I also think that public officals, or others in the public eye, that can be identified by nature of their voice are fair game by nature of their positions.
Bottom line, if the subject cannot identify themselves, you're safe.
Will this work for you?
Majik