You'll note the second amendment says "Congress shall not", it says what Congrss may not do. It does not claim to give citizens any right at all. Instead, it says Congress may not abridge THE right. Not that they must give you some new right, but that they must not violate THE right, the right you already have, by virtue of being human.
The plain wording of the Constitution simply recognizes that you have these rights and the government shouldn't violate them. Nowhere dies it define what exactly "the freedom of speech" is. Perhaps the reason the founders didn't feel the need to define these rights is because they were already defined in the existing law, English common law. I think you'll find that at the time they wrote "THE freedom of speech", they understood that freedom to include unpopular speech, but not shouting fire in a crowded theatre, libel, and a few other things.
This fact, that the ConConstitution speaks of protecting pre-existing rights, is crucially important. If your rights were not pre-existing as part of bring human, they must have been given to you by government. What government can give, government can take away. The Constitution rejects that view. Because your rights are endowed by your creator, legal documents can neither remove the nor define them. The Constitution doesn't define the freedom of speech because it can't. If it could, it could define freedom of speech as freedom to say approved things. The definition is elsewhere, as it must be.