This is really not a difficult thing to do yourself, I've veneered lots of things including speakers.
I've had great results using a contact adhesive but many suppliers sell iron on veneer. This place has some basic woods
http://www.iplacoustics.co.uk/INDEX.htm but a quick search of the web will turn up more exotics like zebrano, yew, olive, ebony etc.
How I've always done it is stick oversized pieces on then trim them flush with a scalpel (stanley knives don't leave a neat edge) veneer side down using the speaker cabinet itself as the straight edge. The hardest part is cutting the holes for the drivers, again overlap the holes and trim back using the existing hole as a cutting edge. Sounds hard but so long as you take a bit of care and time over it the results are perfect.
Once all sides are done, a quick sanding paying close attention to the edges is needed. I found sanding all the edges off as right angles made for a very smart and continuous look.
There are a couple of finishing options:
waxing has a matte sheen, is water and red wine resistant liquids just bead on the surface. Its super quick and easy to do and looks smart.
french polishing is also remarkably easy to do (lots of guides on the web) time consuming but easy enough on flat sided boxes like speakers.
If i did a pair again I would get it piano lacquered by a professional finisher.
After you've sorted the finish its time to re install the speakers, you can use the existing cabling but this is a good time to change it for something superior if you wish (its surprising how many are wired with bell wire.
If you do decide to go ahead on your own feel free to contact me for advice as needed.
Rob