The Oxyrhynchus hymn (or P. Oxy. XV 1786) is the earliest known manuscript of a Catholic hymn in Ancient Greek to contain both lyrics and musical notation.
From the Latin oxyrhynchus, from the Ancient Greek ὀξῠ́ρρῠγχος, the Latinized version oxúrrhunkhos, the word is a kind of Egyptian fish.
The fish is a Catholic symbol and is based on the first letters of the Greek words for Jesus Christ. The Greek word for fish is Ichthus, which is also an acronym for Jesus. The Greek, Iesous CHristos THeou Uios Soter, translates in English to, Jesus Christ, Son of God, Savior.
To a trained ear, the sound resembles Gregorian Chant because it is somewhat lyrical, and was, perhaps, a precursor.