The Christmas song is called "Nowell Sing We, and was sung starting around 1450. In Renaissance times (during the time period that Columbus and other explorers were sailing around, Michelangelo was painting the Sistene Chapel, and Shakespeare was writing plays), much of the music performed was by singers in churches-- and this church music was often sung acapella, with harmony parts.
A songwriter friend of mine, Alice Olsen, has created a beautiful recording of this song, doing ALL the parts by herself! She said it took just one afternoon to do! Wow, I'm really impressed. You will have to take a moment to listen to it.
Just a warning-- this song has words from two languages-- it is part Latin and part English. So it has some unusual sounding words mixed in, like when it says, "Now Rex pacificus is come." "Rex
pacificus" means "King of Peace." There are some other phrases like that, too.
Here's a link to Alice's Songwriter Facebook page, where she has lots of song recordings available online. Go to her Music Store and scroll all the way down-- Nowell Sing We is 4th from the bottom. You can listen to Alice's amazing Renaissance song here-- it is not a snippet, but the complete song.