Has anyone ever used any sprays or paints to protect the cabinet of a speaker?

I'd be surprised if they didn't already have a coat of polyurethane.

If not, clear, slow-setting poly will preserve them and leave the aesthetics.

Erik, thanks.  I'll look into some clear poly. 

Indoor speakers are not designed to be exposed to the elements.  Nothing you do will seal the Cabinet completely and avoid accelerated deterioration.

deeznutz, yes I am aware of that.  My intention isn't to try "convert" these speakers into outdoor speakers.  I simply want to minimize any damage that  the elements will inevitably cause.  I only take them out maybe twice a week, and never in the sun.   If they were a lot more expensive speakers, believe me, I would never take them out...maybe.  :)

i've left a pair of (indoor) technics sb55 speakers, which i sprayed with polyurethane, under on an eave on my open rear porch--i've left 'em out that way for 10+ years through snow, rain and shine and they're still going strong ( i have another pair of boston a70 speakers which i've also polyurethaned and left out for years without sonic issue). admittedly, the cabinets don't look good, but they do work fine.

Loomis, that's really amazing.  I don't expect mine to last more than 5 years if I did what you did.  The summer sun we get is brutal for about 4 hours where i usually have them.  But good to know.  

Oh, look for UV blocking too.

Erik, per your suggestion, I found just the ticket.  Rustoleum has a clear urethane with UV.  I thought you were joking about the UV.  So yeah, I sprayed them puppies over the weekend.  They look much darker now, and cooler!  Thanks, man!