One of the differences from the 13-inch and 15-inch MacBook Pros last time would be the speaker grills, in which the 15-inch model would have them on each side of the keyboard, while the 13-inch model did not. However with the new MacBook Pros, Apple has finally given the 13-inch model speaker grills.
Unfortunately it seems like it might have been for nothing and could merely be there for cosmetic purposes. In the teardown of the 13-inch MacBook Pro by iFixit (via 9to5Mac), they have found that the speaker grills on the laptop seems to be there just for aesthetics because in the image above, the orange rectangles mark where the actual speakers are, and the red marks the speaker grills.
As you can see they are clearly not aligned with each other. In fact they are pretty far apart from one another, meaning that if there is any functionality to them, it is minimal at best. That being said, we are not detracting from the sound quality of the speakers. So far reviews have found that Apple has indeed improved the speakers greatly from previous MacBook Pro models.
This means that even if the grills are cosmetic, the sound quality has indeed been improved upon so that should be worth something.