How to protect the wood finish of speakers.

cover them when not in use. sunlight works fast.

Bee's wax works for me, and yes keep them out of direct sunlight.

I think that I have heard of furniture treatments that contain sunscreen, anyone else know who makes it?

Only two viable options. Either cover them up as suggested, or invest in ultra-violet sunscreen film for your glass. I've got an 8' x 8' glass patio door by my system and installed sunscreen film. It cuts down on glare, enhances the energy efficiency of the door, and stops 99% of the uv light that damages fabric, wood, and art. Plus, it actually improves the view. It has an effect like wearing polarizing sunglasses, the sky looks much bluer.

2nd the Liquid Gold.This happened to my Virgos,when I lived in Wi.This restored the lustre.