Pentatonix the  American a cappella group from Arlington, Texas, currently consisting of vocalists Mitch Grassi, Scott Hoying, Kirstin Maldonado, Kevin Olusola, and Matt Sallee has a thing about Christmas. Since they formed in 2011 they have been producing cover versions of modern pop works or Christmas songs, sometimes in the form of medleys, along with original material.  Scott Hoying and Mitch…