The Oxyrhynchus Hymn is the oldest piece of Christian music that has accompanying notation. Above is a setting of the words to new music by Johann Kim. Below is an attempt to reconstruct what it might have sounded like in a instrumental rendition on reconstructed ancient instruments.
You can find a number of related resources – transcriptions, translations, images of the papyrus, and more – around the web. The Chant Blog has gathered a number of them conveniently in one place.