Soundtrack | Actor. Pentatonix is an acapella group that won The Sing-Off (2009). They have released PTX Vol. 1 and PTX Vol. 2, along with PTXmas (Deluxe Edition). The group consists of the trio (Mitch Grassi, Scott Hoying and Kirstin Maldonado), the rhythm of the group comes from Avi Kaplan (Bass and overtone) and Kevin Olusola (Cello beat boxer extraordinaire... See full bio ».