OK researched it a bit more..
Freedom of speech is not an issue here, since that only applies when dealing with the government. It absolutely does not apply to private corporations. So Sterling is in the clear there.
However, assuming Sterling did not consent to being interviewed, then the recorder is operating against California's wiretapping laws. And not only are they commiting a criminal act, but they are opening themselves up to a civil lawsuit to recovery any related damages.
Furthermore, I'm sure it's reasonable that any owner of a franchise in any sports has in the contracts something close to a morality clause. If the clause is violated, I'm sure there are repercussions, financial or otherwise.
So TLDR, Sterling is not entitled to freedom of speech, nor does he have a defense against the NBA. He does however, have the opportunity to pursue damages against those who recorded the private conversation, assuming he did not consent.