Fundamentally, not all communications are speech, because some communications have explicit direct non-speech results.
According to the Supreme Court, not all communications are PROTECTED speech. (They're still speech. They just don't enjoy the First Amendment protections because they're ALSO parts of crimes for which one can be punished - and in some cases (such as threats or criminal conspiracy) the speech is all it takes to commit or be a participant in the crime.)
Because speech is explicitly mentioned as protected in the First Amendment (and anti-government speech is also specifically a necessary part of another protected right - petitioning the government for redress of grievances), the court sets a very high standard for laws making some kind of speech a crime: Such laws may be overturned just because they have "a chilling effect" on protected speech, by making people avoid such protected speech out of concern that it might be prosecuted.
Regardless, Congress doesn't get to pass laws that preemptively muzzle people or block publication. They just get to pass laws to punish them AFTER they speak (or print,
Yelling "fire" in a crowded theater isn't speech.
Funny you should mention that. The phrase "FALSELY shouting fire in a crowded theatre" originated in a WWI Supreme Court decision declaring that distributing anti-draft leaflets to people of draft age was not protected speech.
My favorite approach to "Fire in a Crowded Theatre" was Abbie Hoffman's (when being interviewed in a crowded theatre):
Interviewer: "But surely you don't advocate shouting fire in a crowded theatre?"