Yes, speakers can be re-veneered. However, it is a great deal of labor since everything needs to be sanded down in order to prepare them for veneering. Of course, some speaker shapes cannot be veneered as veneer will typically only bend in one direction - with the grain. If you have roundovers on, say, the top and the side, the veneer would have to bend in both directions and this almost never works. There are other limitations as well. Normally it is easier just to build new cabinets.
So the answer is a qualified yes and we would have to see the specific speakers to know if there are any issues.
- Jim