This sounds obvious, but I've looked all over the internet and haven't been able to find a simple answer telling me where the sound comes out of on a 13-inch Retina MacBook Pro (late 2012). All I can see on the laptop itself are a bunch of vents at the connection between the screen and the keyboard, some longer vents on the left and right sides of the laptop's bottom, and the gaps between the keys themselves. Anyone have a clue?
The speakers on the MacBook Pro 2012 are on the left and right sides between the aluminium case and the battery. The photos below are of the Macbook Pro bottom case assembly. The speakers are highlighted in red.
It seems the gaps around the keys of the keyboard are meant to act as the speaker "grilles", however not the only avenue for sound to exit.
Source: I use a silicone keyboard protector which covers all the keys with a single unbroken thin membrane. If I quickly remove it while listening to music, I notice the sound sharpen and become a little louder.
Added photo of my keyboard (belatedly, sorry) per the request below. It is embarrassingly filthy, which helps make it more visible in the photo :) The silicone mat extends all the way to the beveled edges of the depression surrounding the keys, covering every opening in the top of the enclosure except for the vents inside the hinge area.
The two sets of three slots at the left and right edges of the bottom of the case (mentioned by the OP) do also seem to function as speaker grilles. I still get an acceptable amount of sound from my laptop even with the keyboard cover on, so my assumption is that they are intended to be the primary outlets, however given the noticeable sound change I get with the keyboard cover, I feel that the keyboard's gaps were likely also part of the Apple designers' plans. I am no audiophile; if I can hear it, it is not subtle.