Thermal dynamics...
The hotter an electrical component, the higher the resistance to the flow of electricity. When its cold outside, its better for your amp and voice coils because it allows the components to work harder without overheating.
As said above, sound waves travel faster in warmer temperature, however it will be close to unnoticeable.
The speed of sound is ~331 m/s @ 32*F, and ~347 m/s @ 80*F. This is a change of only 5%
I don't know about you but I never notice a difference between the winter and summer months of my system... A 5% difference in the speed of sound is more than likely negated by the increased efficiency of the electrical components of the system (Amps, wire, voice coil, etc)