Interesting comments so far. The answer is, "it depends."
First off there is no such thing as sealing wood. We rough turn bowls and coat them 100% in a very thick wax emulsion to slow the drying process down so the bowl does not crack, slow down not stop. Even very hard polyester finishes do not stop wood from changing moisture content.
Most speakers are made from MDF which does change moisture content but it's dimensional response is much less than real wood and uniform in direction whereas wood expands and contracts only across the grain. The wood veneer has no problem following the MDF without cracking. However if you get MDF soaking wet you can watch it disintegrate in front of your eyes. There are waterproof versions but they are more expensive so hardly ever used.
There are two basic types of finishes, closed pore and open pore. Closed pore would be the various lacquers and paints. Open pore various oils and waxes. Speakers are Lacquered or oiled. Waxes are never used.
For oiled speakers which get that dry look and obviously have an open grain use Watco Natural once a year. Wipe it on then buff it out. Soak the rags in water and place then in a plastic bag. Remove all the air. There are cases of oil combusting spontaneously. Lacquer finishes are generally gloss on speakers but might be satin. The grain is generally closed but on a rush job might appear to be open. These finishes just have to be wiped off with plain water. If they get scratched you can rub out the gloss finishes just like you would an automobile. It is a bad idea to use wax on either finish as it will build up and look crappy in time.
