Can speakers be stored in a garage?

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.