Speaker refinish

This would be a job requiring a very dextrous and skilled craftsperson. Veneer is very thin, there's near zero material to sand out. But somehow, you'd have to have a clean, unsealed, & stainable wood surface if you're going to retouch. You can strip the old finish off, but that has a host of different problems and challenges. Assuming you can get to bare wood, you then have to be able to mix and apply the stain to match. But the stained wood and bare wood are going to oxidize and change colors again and they won't match in the future. In short, leave it alone. It's not cost effective or likely to turn out as nicely finished as you'd hope. However, maple changes color much faster on exposure to light than you might think. I've refinished several maple tables and they all started to turn darker within days of the finishing work when left in a reasonably well lit location that received sunlight.

My speakers have a "yew" finish. I think that I better leave them alone and just wait and see. They sound great anyways!

I have a pair of ProAcs in the Yew finish. Try simply applying some Pledge (Extra Moisturizing w/lemon oil) - available at any grocery store. Spray it on a clean cloth and then apply (DON'T spray right onto the finish). Apply it liberally, but carefully (careful to ONLY get it on the veneer, not the hardware, any matte finishes, or, especially, the drivers/tweeters themselves). Let it soak in for a week. Reapply. It does a great job of conditioning the veneer (mine was starting to get a little crackly-looking around the knots in the finish) and it will slowly darken the finish slightly, but more uniformly after several applications.
It's inexpensive, and it won't (in my opinion and experience) hurt anything. It's not waxy either, so no discernible buildup.

Just be aware that only oil or wax finished veneer will be affected by what Steveaustin suggests. Most speakers are finished with a catalyzed impermeable finish that no oil or conditioner is ever going to penetrate.

Every year or so I use Scott's Extra Gold Wood Cleaner and Preservative on all my furniture and audio gear wood; never a problem, always an improvement, with regard to luster and color. I'd recommend trying it, being careful not to overly contaminate drivers and other areas not made of wood. The biggest factor, of course, is keeping sunlight consistent on all areas; that will only take time!