(long answer...) The answer depends on what kind of finish Snell put on the speakers to begin with. Most "furniture" produced today is finished in a way that seals the wood quite effectively, and replenishing the finish is usually not an issue. The wood only needs to be kept clean for the finish to continue to be effective. If Snell took an old fashioned approach and only used an oil finish, then, yes, the finish would lose moisture over time, and some replenishing would be needed.
BTW, there is nothing at all inherently wrong with dry wood. Unstable humidity and temperature conditions, not dryness itself, stress wood, and sealing finishes are used to "insulate" the wood from these changing conditions. But these considerations are much more of a concern with solid wood, not veneer.
A natural oil often is applied to a wood like walnut, because it really brings out the depth and color of that kind of wood. If a sealing finish was not applied over the oil finish, then the oil eventually would lose moisture. This may or may not interest you, and is a fine point when it comes to veneer, but the point of applying some more oil would be to replenish the finish, mainly out of cosmetic concerns, not to protect the veneer from drying out or otherwise "condition" the wood.
If indeed Snell gave the speakers a natural oil finish and left it at that, applying some tung oil (a natural oil of excellent quality) would be a great idea. I would try to get some 100% tung oil, just to be most compatible with the original finish. A lot of tung oil finishes have other, varnish-like (sealing) components in them. Nothing wrong with that, and I'm sure they'd work just fine for your application. But if you want to stick with the original traditionalist approach of a straight oil finish (and again, that's assuming that Snell took that approach), consult a woodworkers supply store for 100% tung oil.
If you want to go farther, a good top finish over the tung oil (after it dries, of course) would be wax. This will help prolong the moisture of the oil finish and provide some extra protection against scratches, make dusting easier... the kinds of things wax does. Find a good quality, hard paste wax, heavy on the Carnauba wax as an ingredient. Use a lot of elbow grease to rub it on to get a very thin, hard wax layer, just as you would do with a fine automobile. But this step would be totally optional, since speakers are not going to get a lot of wear. BTW, stay away from furniture "polish" or oils. (Sorry, I disagree with Gmood1 here.)
Some big "ifs" here. Hope this helps.
