Byzantine Music

Byzantine music clearly reflects the Christian Church, it was the "medieval sacred chant" of the Church.
This style of music dates back to the 8th century. There are chants that date as late as th 12-13th century, and those are the ones that are more well known, and can be read in church texts, patristic writings and medieval books.The following website gives samples of Byzantine music from different periods in time. http://www.oud.gr/music_byzantine.htm
Phos Hilaron is an early hymn, from around 4-5th century, and since it is sung in the evening when lamps are being lit, they will sometimes refer to it as the "lamp lighting hymn." The bishop St. Athenogenes was given credit for this vesper hymn. This website gives a bit more info on this vesper(evening praying service) hymn. http://www.smithcreekmusic.com/Hymnology/Greek.Hymnody/Phos.hilaron.html
Here is also a video containing the hymn, with an English translation.

The Gregorian Chant was another form of early medieval music. In was named for the pope (590-604), St. Gregory. This chant became the sacred chant of the Church and went all across Europe. "De Profundis" is a Gregorian chant and the following video is of the chant itself.

Upon listening to both, it is very hard to pick up many differences given that these hymns have probably been altered with time, and that the language is completely foreign. But what you can clearly pick up is that in the Gregorian chant, they do put more emphasis on the vowels, and carry them on for long notes, and throughout a single stanza. The byzantine music is more fast paced, with different sounds throughout, they don't focus on one for a long period of time. Like I said though, it is very hard because if I was to just listen to them once, I would say they sounded the same.
The latter one though, I must say that that is what I hear when I tune to the occasional Christmas Eve mass with the Pope. It has survived in the Roman Catholic Church and the hymns are still sung today. Browsing through some, and the titles, they have many that I do hear when I even go to Sunday mass. (back when i attended mass. )