This video by "Theologica37," part of a "failure of secular ethics" series, makes a decent stab at tracing emotivist tendencies through Hume through Ayer (verificationism, like Carnap) and Stevenson.
Watch on YouTube.
The version of emotivism described up front may well reflect Ayer's position, as I've not read that recently and just don't know, but it doesn't convey the nuances of Stevenson and Hume (as we went into on those episodes). In neither of those cases is morality an entirely subjective matter: Hume's "moral sense" is actually pretty close to Moore's intuitionism, in that the "sense" is supposed to be grasping something that any non-damaged individual is supposed to be able to grasp: it's an objective matter. Stevenson focuses instead on the objectivity of the linguistic: it's the connotation of the terms in public consciousness that conveys the punch of an ethical claim, not the speaker's private feelings. Likewise, the narrator here states that moral sense theory is incompatible with naturalism, whereas our listeners should be well aware after our Churchland interview especially, but also our Flanagan one, that naturalism in the broad sense that Hume was committed to is compatible with talking about levels of organization besides those studied by chemistry and physics: naturalists need not be behaviorists or reductive materialists.
So the video has set up some straw men, and then uses some of the same objections we saw in Moore and in MacIntyre to knock them down: for instance, Moore's objections to the naturalistic fallacy ("I approve of X" doesn't logically imply "you should do X") and MacIntyre's claim that the emotivism can't do justice to specifically ethical talk (he actually quotes MacIntyre around 10 minutes in). The narrator has obviously done his homework, and it would take more time than I'm willing to give to decode all of his claims; his comments on Russell, for instance (whom I've not read in this context), don't at all make me feel well informed about Russell, and I get the impression that in general the narrator is not seriously trying to engage, i.e. explain the issues to, the reader, but rather to throw out selective pieces of jargon so that the listener will instead take his word for it that he knows what he's talking about and so admit that the thinkers he's discussing are crap. Here's a quote from the video: "The issue here is deeply internal to the emotivistic mind game, and it occurs even granting all of the as shown to be rather problematic and contradictory premises and assumptions." This is the point I keep making about the virtues of analytic philosophy: slow down, and don't try to sound smart. Stop and analyze the terms you're using. Consider the apparent difficulties in the way you've formulated your position and say why those really aren't difficulties. Your model should be more like a teacher than a lawyer, even if you're aiming to talk to people that already know a lot of philosophy.
For instance, towards the end he talks about emotivist moral arguments being "objectively invalid" because they have "no objective truth content." These particular phrases are faux technical: validity has to do with premises leading unerringly to a conclusion within an argument, and there's no question of "objective" or "subjective" validity. Likewise, talking about "objective truth" (and hence "subjective truth") is a barbarous colloquialism at best. "Truth" applies to sentences, or propositions, such that a sentence either is true or it's not, and the adjective "objective" is either redundant or misleading. (And though I get the point of throwing in the word "content" there, it's particularly superfluous.)
Objectivity is an epistemic matter; you can talk about "objective knowledge," meaning knowledge that comes from a certain kind of source, or which is verifiable in certain ways; how you define it depends on what your epistemology is. So the narrator here is trying to sound precise and technical as a matter of rhetoric, but in this and many other areas isn't really getting the technical (i.e. as established in actual philosophy classes) down. Much better, then, to adopt a more colloquial style (which is not incompatible with speaking carefully) and bring the listener around with gentle persuasion rather than a debater's hammer.
So, yes, there are problems with the various kinds of emotivism, and some of the things said in this video are right, but overall, he's oversimplifying the issue in just the way that theistic ethicists so often tend to do when trying to brush away the whole of secular philosophy. "I'm having trouble elaborating this theory of how morality can exist in a natural world... so it must be God! That's the only alternative to my half-assed attempt to spell this out!"