Can speakers be stored in a garage?

I have stored speakers and amps in my unheated garage, low humidity, for years with no problem. You should be fine.

It may depend on where you live, I certainly wouldn't in New England if they had any value. 

I would not store anywhere subject to wide temperature swings. Also of course needs to be secure from thieves.

Some desiccant would help keep moisture out.  What about mice?

I would be willing to hook everything up and store them in my listening room.  PM me if interested

Plastic bags, even with desiccant, will not eliminate the effects of humidity. Plastic is porous. With time moisture will seep in.
And, the post regarding mice is quite relevant. I stored boxes in a garage attic, only to find the mice decided to make a winter residence in one of the boxes.
I would suggest finding a storage facility with a climate controlled environment, if possible.
B

The stinkiest lowest grade dryer anti-static sheets that money can buy. 

Put them in the boxes, and that will keep all critters away. They hate that stuff. 

It'll drive wasps, bugs, mice, squirrels, raccoons, etc, right out of an attic.

It's flat out toxic. Never use those dryer sheets around humans, essentially. 

Keep the boxes off the ground, away from fluctuating temp and humidity, and you might be good for a short while. I usually put a 1000 watt heater in the garage if I have to store audio there. Just to keep from freezing and to keep the moisture down. The heater also keeps things away from the dreaded dew points. A couple of cycles are no big deal. Repeated cycles are the problem.

Best to stay away from the garage and go for climate controlled storage facilities.

Why not just hook it up inside, in a less than ideal spot, and enjoy it?

This is far from ideal for storage speakers may go bye bye.

Yes I have and I'm on Long Island ,NY our temperature go from freezing to very hot weather..Just keep them off the ground .I have had stuff in garage, basement's and attics with out problems.