Great topic Snake!!!
Looking up the "exact" definition, it is pretty much I assume what we all think:
Treason: "Violation of allegiance toward one's country or sovereign, especially the betrayal of one's country by waging war against it or by consciously and purposely acting to aid its enemies."
Given we are fortunate enough to have freedom of speech in our country, and knowing that some of that free speech may not please everyone, I think, for the most part "speech" is merely words.
HOWEVER, our freedom of speech "would" be treasonous as in DV's example (and the most known definition) of giving secret info to another country; and although I'm not sure if it would fall under treason or not, of course threatening to kill your President wouldn't be included as freedom of speech, as in threatening anyone's life.
I guess anything one might intentionally say with a purpose of malice,to cause harm to, or betray one's country would be considered treasonous. I am curious though for other examples of "Speech" being treasonous.
TIS