Here's the chant score, from the Brazilian Benedictines:
The text is from Wisdom 11:24-25,27, and from Psalm 57.
JoguesChant offers this translation:
Your mercy extends to all things, O Lord, and you despise none of the things you have made. You overlook the sins of men for the sake of repentance. You grant them your pardon, because you are the Lord our God. Be merciful to me, O God, be merciful to me, for my soul confides in you.
Here's a video labeled "Feria IV Cinerum 1 - Introitus - Traditional Latin Mass" (i.e., the "Extraordinary Form"); it's labeled "Miserere Mei," but I think that's just an error:
The Collect for the day echoes the portion of Wisdom in this Introit:
Almighty and everlasting God, you hate nothing you have made and forgive the sins of all who are penitent: Create and make in us new and contrite hearts, that we, worthily lamenting our sins and acknowledging our wretchedness, may obtain of you, the God of all mercy, perfect remission and forgiveness; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.
Hatchett's Commentary has this about the collect:
The Sarum collect for Ash Wednesday can be traced to the Gelasian sacramentary where it is the inital collect for the fall ember days (no. 1037). In the Gregorian sacramentary it is used on the Wednesday before the first Sunday in Lent (no. 154). It might be translated:
Grant, we beseech, Lord, to your faithful people, that they may undertake the sacred solemnities of the fasts with fitting piety, and that they see them through with undisturbed devotion.
Cranmer's new collect for the 1549 Book places the emphasis on penitence rather than upon the fast. His preamble is similar to that of the form for the blessing of the ashes and to the introit appointed for that day in the Sarum missal. The petition is obviously inspired by Psalm 51 which has traditionally been used on this day. We pray for "remission and forgiveness." The word "remission" normally refers to debst, and "forgiveness" to offenses.
The rubric following the collect was inserted in the 1892 revision to insure that the proper of Ash Wednesday rather than that of the preceeding Sunday would be used on the follow THursday, Friday, and Saturday. Such a rubric is also found in the Scottish Book of 1637. From 1662 until the 1928 revision the Ash Wednesday collect was to be repeated throughout Lent daily, a custom analogous to the use of memorials after the collect of the day late in the medieval period. The 1928 Book qualified this by adding "until Palm Sunday."
The rubric referred to above is this:
"This Collect, with the corresponding Psalm and Lessons, also serves for
the weekdays which follow, except as otherwise appointed. "