Someone whose e-mail I can't find again pointed me to this video and rather than explain it myself, I'm going to steal the explanation of the fellow who posted it, David Rowen…
This music video is the culmination and fulfillment of a small dream I have had since college. I attended James Madison University, where I sang in an a cappella group called The Overtones. During my last semester of school, I was commissioned by the American Heart Association (AHA) to arrange an a cappella medley of music from Wicked, The Wiz, and The Wizard of Oz for the Overtones to perform at two Oz-themed galas hosted by the AHA. Outside of these events, the medley was only performed one other time, and since then, I have wanted to revive and revamp the arrangement, and take the creative vision to new heights. With the help of an amazing team, a stellar cast, and generous donors, the new vision has finally come to fruition. I am thrilled to present "The Oz Medley."
I love this kind of a cappella singing. I suppose I'm fortunate that I have a wretched singing voice because if I could sound like these folks, I'd join one of those groups and never do anything else. This is worth ten minutes of your time. Thank you, whoever sent me the link!