Winter, dry air and carpet = static electricity. If you touch the case metal while holding the mp3 player this shouldn't happen again. Case should be grounded.
Just for grins, I'd check the ground on the wall socket. There are a thousand ways to do that, one is to spend $10 on a surge protector that has warning lights for bad ground like this one. http://www.amazon.com/Belkin-SurgeMaster-F9H620-CW-Wall-Mount-Protector/dp/B00006BBAC/ref=sr_1_3?ie=UTF8&qid=1326678683&sr=8-3
(I pciked one at random, this may not be a good one, but you can see what I mean)
Agree with Phyco that this is 99+% not a problem.