speaker wood care

S7horton is both right and wrong. First and foremost avoid large changes in ambient humidity. Too wet is usually the case, but too dry can be just as bad. When the moisture content falls below 6-7% the cellulose bonds within the wood become very brittle and can crack (could affect the sound in something that relies on box reverberation). As for applying something, a lot depends upon the finish type and none of them will add moisture nor will anything keep the speakers from losing moisture in order to reach stasis with the environment. With that said, it is hard to go wrong with a good paste wax such as Johnson's (not auto wax) to protect the finish. Ultimately, you should consider adding moisture if it is really that dry. FWIW, an air washer (filter) such as a Venta should work great for you.

Disclaimer: I am a cabinet maker

Isn't Las Vegas in the desert? Would that not be considered "extremely dry" ? I use a Venta here in Michigan during the winter as my boiler severely dries out my home, and the winter air does not contain enough moisture to help. Many older furniture factories have steam misters to help add moisture carefully.
As for "sealing" to prevent transmigration of moisture- doesn't work and never will. Furniture making would be 10x easier if that were the case. Sealing only slows down the process- a lot, I admit - but nature seeks equilibrium. Lemon oil on the other hand, is mainly solvent with a little oil- a nice penetrating "cleaner" offering little protection. Wax will not keep moisture in/out, but clogs the micro pores/cracks a bit and ads a slippery protective coating. Both beneficial for longevity of any piece of fine woodwork. You might try asking a known skilled furniture maker in the vegas area, they will know. For the record, wax won't hurt anything. If you don't like it, lemon oil will probably take it right off ;).

Oh BTW, FWIW I've seen veneer over mdf crack due to severe dryness. Higher quality items will have a layer or two of "cross banding" (veneer with grain rotated 90 degrees) seldom seen in audio products. Really though, if your home is very dry you might look into some form of humidification. Your sinuses will thank you as well.