Boot up the MBP.
Turn it around.
Look into the headphone jack.
Do you see a "red light" on in there?
If you DO see it, that means the jack is "set" to optical-out, and is providing a digital signal to the port, even though there's no headphones plugged into it.
So, the speakers won't work, because the Mac "senses" there's a connection in the headphone port (even though there isn't), and switches the speakers OFF.
If the port gets "stuck" in the "digital-optical out" mode, now ordinary headphones won't work, and the speakers won't work, either.
Here's a link to an article that "unsticks" the port the old-fashioned way -- by inserting/removing a headphone plug several times:
So it seems I've done something to mightily mess up my new MacBook Pro. The other night while in my hotel room at AstriCon, I was using my headset for some Skype calls and then had it in when I...
www.disruptivetelephony.com
But if that doesn't work for you...
Some years' back a MacRumors user figured out how to disable the optical-out signal using software.
Read this post:
Also see this post:
My 2009 MacBook’s speaker doesn't work (most times). The digital audio out port gets stuck (a red glow appears from within the socket). It is an old MacBook with separate headset & mic ports....
apple.stackexchange.com
Scroll down towards the bottom to see "the software fix".
If this works for you, please save this thread and tell us how it went...