I think you're confusing something along the lines of "parliamentary privilege" for freedom of speech. Various forms of government have imposed as part of their decision making process a duty to listen to various forms of official speech. That's not a component of freedom of speech though, rather it's a component of that specific form of governments process.
I'm not sure where you find a duty to listen in Athens, my best guess is you are pointing to the duty to attend the assembly, which fits this form of "parliamentary privilege" rather than a broader duty to listen. To contrast, by my understanding political commentary in the form of plays and the like were quite common in Athenian culture, and I'm fairly sure there was absolutely no duty to pay attention to those. I'd also note that even with the assembly "the crowd might raise a clamor and refuse to listen to a speaker advocate an unpopular proposal" (your source) which suggests little duty to actively listen even there.
The protestation of 1621 is directly related to parliamentary privilege (and why I labelled this the way I did, though I don't think the label fits perfectly, since it applies to many smaller government meetings than parliament).
I can't say I'm familiar with the first amendment debate you are discussing.
> You restricted it to "government intervention, not...private agreements."
I don't think this is the cause of our disagreement, but I'll grant you that that phrase was slightly inartful. E.g. there are authorities other than governments, like a local gang, who could impinge on freedom of speech with threats of retaliation, and to the extent that private agreements are enforced by an authority it's possible for them to do so.