The Sequence (with music!)
There are a few major feasts in the church year that are distinguished by the addition of a Sequence to the liturgy: Easter, Pentecost, Corpus Christi, and Our Lady of Sorrows. What is a Sequence, and what is it for? It's a piece of theological poetry, based in scripture but not directly taken from it, which forms one of the lessons for the day. I like to think of it as a kind of homiletic fail-safe - no matter how good or bad a preacher your pastor is, you will get this one elegant lesson that encapsulates all you have to know about the feast. In the Ordinary Form, it is sung between the second reading and the Gospel Acclamation.
While two of the Sequences are optional, the ones for Easter and Pentecost must be at least recited - but they are so much better when sung. Since the Sequence is a lesson, similar to the readings, I think it is best to sing it in the vernacular - at the very least, the congregation should have the translation in their hands. If it is not printed in the missal that your parish uses, and you must sing it in Latin, direct their attention to the translation you have placed in the seats, or read the translation before singing. It should be proclaimed clearly, not too slowly, and with minimal distracting elements - the accompaniment (if any) should be limited to chords and the melody. Rather than harmonies, use alternation of voices if you really must have variety; each Sequence has a more-or-less repeating pattern of music which can be sung antiphonally by two cantors, by men and women, or mixed choirs standing in different places (switch at the double bar lines). The typesettings I have linked here are the vocal line only.
The Sequence for Easter (Latin text Victimae Paschali Laudes, or Christians, to the Paschal Victim) has a range of an octave and a fourth, with the low note being a quick dip, so if it's a bit of a stretch for your higher voices, the low voices can cover - definitely err on the side of a little too low rather than a little too high. It has a narrative section which is a great spot for alternating between men's voices and women's, or even a solo woman singing the "part" of Mary (she will need to cover the full range of the piece). Normally, "dramatic readings" are out of place in the liturgy, but since this is straight-up Poem, with a cue and a response, like a play, it's free game.
Veni Sancte Spiritus, the Sequence for Pentecost (not to be confused with Veni Creator Spiritus, which was also written in the Sequence format, but does not enjoy the distinction of being an official part of the liturgy), has a much more regular verse pattern - verse one and verse two have the same music, verse three and verse four have the same music, and so on for ten verses, perfectly regular. The range is one octave plus the leading tone at the bottom - easily in range if you pick the right starting note. Watch out for one extremely derpy element, thanks to the USCCB - if your translation has "updated" the "archaic" language which modern humankind is apparently incapable of understanding, you will have this egregious couplet:
Clunk. The rest of it rhymes beautifully, this blot sticks out like a wasp in your ice cream. It's an easy fix.
Now. The "optional" Corpus Christi Sequence (Latin text Lauda Sion Salvatorem, or Laud, O Zion) is an incredible hymn written by the master of tightly-packed theology: St Thomas Aquinas himself, no less. Difficult concepts are deftly combed into strands of poetry and woven together into a complex braid of understanding. It is very long: twenty-four verses, each pair a little different from the pair before, and sometimes in confusing order - verses one and two go together, three and four, but then five and six are echoed by seven and eight, and let's not even get started on the metrical shift at the end. Because of the length, you've got to keep it going at a good clip, maybe a tick slower than speaking speed. With a range of an octave and a fifth, I've never had the guts to ask my whole choir to sing this one, but always sung it as a solo. Getting the right starting pitch is crucial. A little too high, and you'll start out cruising just fine, but you'll be squeaking and panting by the end. I've set it here so that the highest note is D, because I am an alto who can sometimes sing tenor - dropping to that G below middle C is no big deal for me, but high E-flats are saved for a good day when I'm not tired or nervous, and I'm always nervous before I sing this one.* You know your own range, feel free to start wherever works for you.
(There is a way to sing the "short version" of the Corpus Christi Sequence - just begin at verse 21 (marked "Short version" in my copy, which is its own hymn, Ecce Panis Angelorum. Often the priests at my parish will ask for the short version at the early and late Masses, but will relent and let me do the full Sequence for the choir Mass. It is, I guess, preferable to skipping the Sequence altogether.)
Please, let me know if you used any of these settings! Or, if you ask me to, I will gladly re-set them in another key if you need it. Post in the comments below with your own tips or questions.
*If Triduum is the high point of the choir's year, the Corpus Christi Sequence is the Cantor's Final Exam. Last year, I sang it at a newly ordained priest's first Mass, and his Music and Liturgy professor was there - talk about a tough audience! He approved, however, so I guess I passed!