I've used pure lemon oil furniture polish for years with no detrimental effects to the speakers and it keeps the wood veneer moisturized and clean.
I've been building and selling custom furniture of all types for a living for over forty years and have yet to have a problem using lemon oil, especially on veneers. Veneers can start to split when they get too dry regardless of the finish on them. I have veneered pieces more than forty years old with the original finishes that have been wiped down regularly that show no noticable change in the appearance of the finish. I would be more worried about getting the lemon oil on the cones and surrounds of the speakers.