Very cold temperatures will certainly affect the frequency response of transducers (speakers, headphones, microphones, phono cartridges) but almost always negatively, i.e. high-frequency response usually suffers. In speakers, cold will thicken the ferrofluid liquid that surrounds the voice coils and inhibit the movement of the voice coil, slowing it so it doesn't respond fast enough to high frequency movement, and the speaker will sound muffled or muted. Similar effects may occur with damping materials in microphones and cartridges, but it's odd that you find the sound from your iPod ear buds "crisper" in the cold.
I've not seen any conclusive data on the response of our eardrums to cold, other than the speed of sound changes with humidity and in the cold the air is very dry, but that still wouldn't support your experience.