I don't think this is rocket science. Oxygen is more soluble in cold liquids. Because of this the gas bubbles that form when a knuckle is cracked is likely more rapidly resorbed in a cold joint. Also, bercause of this increased solubility more gas can be dissolved in the cold joint. As a result of this in the cold you should be able to crack a joint more frequently and perhaps more loudly (assuming more gas relates to a louder crack).

I believe there is something to this. I broke my big toe years ago. After it healed the joint cracked very easily for several years. It was markedly different than the other big toe.