Location of Speakers on MacBook Pro Retina, 13-inch
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.

Left Speaker:


Right Speaker:



The best place I have found on the web for information like this has been ifixit.com:  MacBook Pro 13" Retina Display Late 2012 Repair