Libera is an all-boy choir group from London, England. They active since 1984, but seems like begun to known by the world since 1995 - present. Their voices are always called heavenly voice or angel voices. You can know more about Libera from Wikipedia or visit their YouTube channel. (use the link)
"The sound is well, Libera-like, a sound which is both ancient and modern." - Libera
Here are two of their music videos, Carol of the Bells (2015), and Time (2010).