The Hi-Lo's were a four-member vocal quartet formed in 1953, so named to reflect the vocal range of the group's singers. They were led by Gene Puerling, who became known for his densely-layered arrangements for a capella groups. He went on to work with similar groups such as The Manhattan Transfer and Take 6, and won a Grammy award in 1982 for his arrangement of "A Nightingale Sang In Berkeley Square" as performed by The Manhattan Transfer. Sadly, he passed away in March 2008.
Suddenly It's The Hi-Lo's was released originally in 1957, and features a number of jazz and pop standards, as well as a version of "Brahms' Lullaby", arranged by Puerling. Much of the material is a capella but they are backed up by an orchestra led by Frank Comstock, a well-known composer and band leader who worked extensively in TV and film.
For more information on the Hi-Lo's, visit their still-functioning website here. If you have your speakers on, you will be greeted by the group singing "Welcome to the Hi-Lo's ... dot com. BOP!"