Saying that, I've had a MacBook before I exchanged to the MacBook Pro and the speaker quality on both is massively better than the MacBook/MacBook Pro models than preceded them.
The MBP speakers are a little better. They are louder and have a little more punch, but overall the MB sounds really good. Much better than the previous gen.
They are "slightly" better on the Pro, but don't expect bass, midrange depth or anything less than "tinny" on either. I came from a Macbook last year to a MacBook Air, to the new Alum. Macbook 2.4 and the new one is the best of the bunch. Some Windoze laptop computers have 5.1 sound, and sub woofers - (when they work). But, that's why we use Macs, and have Bose (and other) external speakers when needed!!
For the size and weight difference, I choose the 13" Macbook over the Pro.