i don't know the answer to your question but why not external? and then if you're on the go, some IEMs or other headphones? might be cheaper and higher quality
Grab a nice set of earbud headphones then. There are plenty of good choices out there, just make sure you find a set that seals nicely in your ear canal. Laptop speakers in general, especially in <15" models sound pretty bad, it's just a space constrain issue.
The speakers in your computer are custom-shaped, and ridiculously small.
I don't think you'll find replacement non-Apple speakers for your machine, and in all truth, they do a pretty good job given their size. They're about 2x to 3x times the size of earbuds.
If better quality sound is important to you, test-drive the 15" MacBook Pros, or look in to ultra-portable USB speakers.