Here are a few examples of english language plainchant (also known as 'gregorian') from the official books and texts approved in the western rite vicariate of the antiochian orthodox church. Most of what is currently on that youtube account being from the divine office. Most of the sources are based on adaptions by Anglo-Catholics in the earlier 20th and 19th c. such as George Herbert Palmer and Charles Winfrid Douglas, who were when it came to music and liturgical texts nearly 100% orthodox and reflective of the ancient latin words and meanings, despite being officially being within a heretical protestant communion.
Most of the music for the mass comes from these sources:http://http://musicasacra.com/books/ordinary_of_the_mass_english1933.pdf
the filioque always being crossed out and omittedhttp://musicasacra.com/books/plainchant_gradual_1-2.pdf]http://http://musicasacra.com/books/plainchant_gradual_1-2.pdfhttp://musicasacra.com/books/plainchant_gradual_3-4.pdf]http://http://musicasacra.com/books/plainchant_gradual_3-4.pdfhttp://musicasacra.com/books/introits_sarum.pdf]http://http://musicasacra.com/books/introits_sarum.pdf
Eventually in 2012 a book of the all most frequently used sequence/proses for the mass of the 11th-13th c period will hopefully be published by Lancelote Andrewes press with the texts from this book in english:The liturgical poetry of Adam of St. Victor, Volume 1-3
, with accurate english metrical translations by Digby S. Wrangham http://books.google.com/books?id=g4uBseHbrpsC&oe=UTF-8
most but not all of them use the music found in the book "Les Proses D'Adam de Saint-Victor
by Pierre Aubry (1900)" http://books.google.com/books?id=n647AQAAIAAJ&
Comments about the mistaken removal of sequences from western papal (RC) liturgy after the protestant reformation can be read here: http://danielmitsui.tripod.com/aaaaa/seq0.html
tens of thousands of sequences, a large portion of the Roman Catholic musical tradition, were simply discarded and forgotten.
These two books contain most of the antiphons on the magnificat for the office:http://musicasacra.com/books/salisbury_antiphoner.pdfhttp://www.allmercifulsavior.com/Liturgy/SarumPsalter.html
The Sarum Psalter put together by Fr. Aidan Keller is the best book for ferial day to day office antiphons/responsories and commons of the apostles/confessors etc. Having official approval for usage within ROCOR. Fr. Keller carefully based his work on that found within Dr. William Renwick's Sarum books in Latin. He made no changes whatsoever in arrangement or rubrics of that used in latin for as long as anyone remembers, except for occasionaly translations of hymn texts to be more in conformance with Orthodox theology (rarely a problem).
The Monastic Diurnal Noted is however the official book for the antiochians vicariate, however there is no objection to supplimenting what it contains with office music from the above two sources generally speaking. It is not available for free as the others are, but it contains the best source for varying propers, sanctorale and temporale. Which is what Fr. Aidan's version sorely lacks. Using both together gives the best results (there are no serious contradictions between them).http://www.andrewespress.com/mdn.html
I myself am currently fusing the music and texts from both Sarum Psalter and Monastic Diurnal with the Challoner Douay Rheims (possibly a Coverdale version mostly likely afterward) translation of Psalms into booklets to be able to be conveniently sung by a novice without resorting to using 3 different books at once (one for music, one for psalms, one for psalm tones, etc). This is one of the reasons why the divine office has difficulty being sung very well at many parishes or churches of the western rite at this time.
Along the lines of what Dr. William Renwick does here:http://www.humanities.mcmaster.ca/~renwick/sarum-english.htm
Most of the hymns for the office from the monastic diurnal are contained in "The Hymnal Noted" by John Mason Neale, published in 1854http://www.lulu.com/product/paperback/the-hymnal-noted/13031119
If one learns to use all these sources one begins to realize the western rite can indeed have a very full beautiful range of music.
Few parishes currently use all these books, due to the fact that most of them have extremely limited abilities musically at present, but the future is bright. Eventually every western rite parish will have at least one precenter/cantor who can sing much of these on a regular basis, as one typically expects in eastern rite churches.
One of the final missing aspects from the western musical heritage is the proper responsories for matins, and proper antiphons from the rest of the divine office for full range of feast days, such as St. Vincent and St Jacob the great, as found
Finally the various tropes of both ordinary and proper chants of the mass will need to be adapted to english eventually, as well as being available in latin as well, this will be one of the more challenging tasks due to the localized nature of their usage. Most likely focusing on the Tropes used around Salisbury, England, Dublin Ireland and Paris, France is the best solution, as the english, irish and french currently form the nucleus of which most western rite parishes ancestors come or identify with. In other words an updated version of the Plainchant Gradual and Ordinary of Mass with the tropes added into them will eventually be published. (Much like Sequences, Tropes were suppressed at the Council of Trent at the height of the protestant reformation influence, they contained rich theology the equivalent of kontakions/troparia, much like the sequences.)
The Sequences of Notker of St Gallen from the 9th century have not had much attention either, several of them are included in the Sarum Graduals directly before the 16th c. Reformation.
One may find most of them notated in latin in both modern and neume notation in the following two books:
Adapting them into english is one of the greater challenges, but not impossible.Early medieval chants from Nonantola: Part IV: Sequences
by Lance W. Brunner (1993)The Early Medieval Sequence
by Richard L. Crocker (1977)