Music: Three sheets of Gregorian Chant

The New Liturgical Movement is also watching this.

[Update: thanks to the helpful comments below (and at NLM), I'll be expanding this blog post to include the explanations for the "incomplete" parts of the sheets (such as versicles that seem to stop mid-word).]

Below you'll find images of three double-sided sheets of vellum with Gregorian Chant. I bought them at the Golden Nugget antique market (just outside Lambertville, NJ) this past Sunday, for a mere $150. They measure about 20" by 11" and are in varying states of disrepair.

Sheet 1A-B
The drifting et clamate at the beginning of this page will be explained later. The front of the first sheet includes references to Isaiah 7:14 (Ecce, virgo concipiet), Isaiah 9:6 (Admirabilis deus), Isaiah 9:7 (Super solium), and Exodus 4:13 (Obsecro inquit), and then a text I cannot find in the Bible (vide afflictionem) which continues to the back of the sheet.
... et clamate. V. Salva: R. Ecce
virgo concipiet et pariet filium dici-
t dominus et vocabitur nomen eiu-
s. ?. admirabilis deus fortis: ?.
Super solium david et super re-
gnum eius sedebit in eternum.
?. admi: R. Obsecro inquit domine
mitte quem missurus es vide affli...

The back of the first sheet completes the phrase from the front. It also contains a reference to Psalm 105[104]:4 (Memento nostri) and Isaiah 49:13 (Laetentur caeli).
...ctionem populi tui. R. Sicut locu-
tus es veni et libera nos. V. Me-
mento nostri domine in beneplacito
populi tui visita nos in salutari tu-
o. R. Sicut: ????? Allelu-
ya alleluya alleluya: seculo?:
Laetentur celi et exultet
terra jubilate montes laudem quia...

Sheet 2A-B
This sheet has larger margins than sheets one and three, so I suspect it is from a completely different manuscript. I can tell the front from the back because of a few clues. The first image (the front) has an F-clef and is clearly continuing from some other page, whereas the second image (the back) has a C-clef at the bottom and is finished with its last word. In addition, the first image (the front) has a C-clef at the bottom, and the custos -- the fragment of a neume (or note) at the end of a staff -- is a "C", which matches with the leading "C" neume of the second image (the back). This page appears to be part of a requiem.

The front has three initials in the upper-right corner (SLO or JLO). The text starts by completing what I assume is an alleluya. Then it refers to Sirach 45:9 (Induit eos dominus) and the apocryphal work 4 Ezra 2:35 (Lux perpetua lucebit).
...luya. V. Induit eos
dominus stola glorie et corona-
vit eos. P. Indie. R. Lux
perpetua lucebit sanctis tuis do-
mine et eternitas tem-
porum alleluya alleluya.

The back begins with Psalm 117[116]:15 (Vox exultationis) followed by a reference to Revelation 11:4 (Isti sunt due olive). The last word on the page is Habent (with the n omitted) which is the continuation of the antiphon (habent potestatem claudere...).
Vox exultationis et
salutis in tabernaculis iu-
storum. P. Et eter. R. Isti sunt
due olive et duo candela-
bra lucencia ante
dominum alleluya. Habent...

Sheet 3A-B
Given the wear at the bottom-right corner of the first image, I've assumed that's the front. It (the front) begins with a fragment of Luke 1:34 (virum non cognosco, Mary speaking to the angel Gabriel); in the middle there is a brief excerpt of Micah 1:3 (Domino egredietur) followed by Philippians 3:20-21 (Salvatorum expectamus); it ends with Titus 2:12-13 (Sobrie et iuste et pie) which is continued on the back side (vivamus in hoc seculo).
... virum non cognosco et respondens
angelus dixit ei R. Spiri:
In secdo noctno ?? alla? Domino. V. Egredietur Do.
Salvatorum expectamus
dominum ihesum xristum qui reform*a-
bit corpus humilitatis nostre confi-
guratum corpori claritatis
sue: V. Sobrie et iuste et pie...

The back refers to Jeremiah 31:10 (Audite verbum domini gentes) and has the phrase Salvator noster adveniet; it concludes with Jeremiah 4:5 (Annunciate et auditum facite loquimini). From what Google has told me, the next two words of this phrase should be... et clamate! Which are the words on the first page I deciphered! I have two consecutive sheets!
...vivamus in hoc seculo expectantes bea-
tum spem et adventum glorie ma-
gni dei: qui refor*. R. Audite
verbum domini gentes et annuncia-
te illud in finibus terre et in in-
sulis que procul sunt dicite: P. Salvator
noster adveniet. V. Annun-
ciate et auditum facite loquimini...