The song played by Dwight Schrute in the Christmas special episode (Christmas Wishes) of The Office is the Trans-Siberian Orchestra's "Carol of the Bells".
This Christmas episode has a lot of things that make up a good Office episode at play here. Jim and Dwight were in a pranking zone, there was a whole lot of Erin Hannon, Andy Bernard wasn't ridiculously annoying and of course there was an amazing montage/air band; ate and Gabe joined Dwight on air guitar, air keyboard, and air bass, respectively.
Not only were Jim and Dwight back to doing what they do best, pranking, but it wasn't the same old song and dance. According to tvfanatic.com, Jim and Dwight were pranking themselves, which made it that much funnier.
Tip! You can view pictures, get episode information, cast, join the conversation and more about The Office at TV.com.