Only read "External" so far, but I propose god(s) be divided into "trusted and idealized authority figures", "internalized sense of commitment to integrity of respected and admirable reputation (honor)", and "external personification of inner conscience".

If people cite God as the source of spiritual value, it's because he represents a combination of these things and the belief that their values are ingrained in reality. God isn't the root cause, and taking Him out of the equation still leaves the relevant feelings and commitment.

Also, "other humans" isn't relevantly different from "other agents".

Also, also, I'm not entirely clear on the point of this post (probably should've brought that up before correcting you, really). Are you citing actual sources of value, or the things people sometimes believe are the sources of value, whether or not they're correct? Value is necessarily formed from concepts in the mind, so the brain can be assumed to be the thing most usefully termed the origin.

Also, also, also, when you say "value" do you just mean moral value, or things people care about on the whole?