"Freedom of speech, also called free speech, means the free and public expression of opinions without censorship, interference and restraint by the government."

https://en.wikipedia.org/wiki/Freedom_of_speech_in_the_Unite...

(Emphasis mine).

Specifically: "The First Amendment's constitutional right of free speech, which is applicable to state and local governments under the incorporation doctrine,[6] prevents only government restrictions on speech, not restrictions imposed by private individuals or businesses unless they are acting on behalf of the government.[7]"

So companies can restrict your speech all day long, as happened with James Damore.

"However, laws may restrict the ability of private businesses and individuals from restricting the speech of others, such as employment laws that restrict employers' ability to prevent employees from disclosing their salary to coworkers or attempting to organize a labor union.[8]"

But that is not the 1st amendment or general FoS legislation, but specifically employment laws. And employment laws are notoriously employer-friendly and employee-hostile in the US.