And where is corporate speech protected in the Constitution?
Nowhere. But then, neither is anyone else's:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
The 1st Amendment simply prohibits Congress from abridging freedom of speech. It makes no distinction as to whether the speech originates from people, corporations, chimpanzees or disembodied spirits. Congress shall make no law abridging the freedom of speech. Period.