No, Flynn's martial law rhetoric isn't sedition. But it isn't necessarily legal either.

Criminal laws exist to disincentivize and control behavior that a stable, ordered society deems intolerable. Because indiscriminate murder, rape, assault and theft stoke chaos, fear and more violence, for example, those actions are banned and can send violators to life in prison or even the death chamber. Although normally criminal laws target dangerous actions versus speech, the kinds of unbridled speech rampant in cyberspace have increasingly opened up new frontiers for violence.

The First Amendment protects free speech, but in 1969 the Supreme Court held in Brandenburg v. Ohio that “the constitutional guarantees of free speech and free press do not permit a State to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.” (Emphasis mine.)

Martial law is not mentioned in the Constitution. Nor is it authorized by any act of Congress. The Supreme Court has never directly held that the federal government has the power to impose martial law. Although the Insurrection Act allows the president to use armed forces to “suppress” an insurrection and restore immediate law and order upon the request of a state legislature or governor, an 1878 law called the Posse Comitatus Act otherwise forbids the use of the military for domestic law enforcement. A criminal statute puts members of the military who prevent or attempt to interfere with voters “exercising the right of suffrage” at risk of going to prison for up to five years.