Peter Hollens and Pentatonix both have some good versions of Mary Did You Know on YouTube. Their performances of the beloved Christmas song are highly popular and the official videos on YouTube each have millions of views.
Peter Hollens performs his version of Mary Did You Know a cappella while sitting in a church. The song gradually builds toward its climax as he sings harmony with himself and adds vocal instrumentation.
Pentatonix also performs this song a cappella. Standing inside a cave and holding candles, each member of the band takes a turn singing the melody while the others harmonize and add vocal instrumentation.