It could just be that it has a few coats of clear on it to make it that smooth.
The issue is that if you were to just paint the current surface black, you would probably end up with a pretty smooth surface, but it wouldn't have any grain to it.
What you'd need to do is completely disassemble the speaker, and then sand a lot to get the surface very smooth. You'd need a coat or two of primer, and then coat or 2 of black, then a coat or 2 of clear.
All these coats will need to be sprayed, with sanding inbetween each coat.
I would recommend checking with a local autobody shop, because a spray paint can has a very limited range which makes it difficult to evenly apply the paint. Proper spray guns powered off of air compresors (like an average autobody shop has) provides a much better finish.
I'll post a pic of tower speakers I just finished painting: I used a roller for the primer, and because of that, the finish isn't smooth. Also, because I used a rattle can, it's not a very even finish, but for my purposes, it's plenty good enough. I might work on them further, but considering the painting both speakers requires a whole rattlecan, maybe not
