hmmm, very hard to do. To get a good piano gloss finish you need a compressor, a good paint sprayer, lacquer, a good smooth cabinet and a very clean place to paint it. I have achieved ok results with some gloss black enamels and a very smooth roller. You have to play with thinning the mixture so that it smoothes out before drying. You will never get it piano smooth, but if your surface is really smooth, the roller will leave a slight texture that does not look half bad. Practice on some smooth pieces until you get the desired look before you start on the speakers.
The only other way is to use some good simulated black lacquer wood finish formica and just refinish them. That is if you don't have rounded edges on the cabinets.