Here's an mp3 of this Introit, from JoguesChant; their translation of this text from Psalm 33:5-6, then 1:
The earth is full of the mercy of the Lord, alleluia; by the word of the Lord, the heavens were established, alleluia, alleluia. Rejoice in the Lord, O you righteous! Praising befits those who are upright.The chant propers for today were once used on the previous Sunday; I'm not sure why everything got moved back one week, but will try to find out.
These days, this is "Good Shepherd Sunday," reflected both in the reading from John and the Alleluia and Communion Song which both use the text Ego sum Pastor bonus - "I am the Good Shepherd."
Here's the chant score for the Alleluia:
Here's a video labeled "Domenica IV di Pasqua - Alleluja" - but it's definitely not the chant in the score above! As far as I can tell - by going to the YouTube uploader's page - it's an Ambrosian chant version of the Alleluia. And it is beautiful!
That was recorded in 2008; here's the same chant from 2009:
I must get to know more about Ambrosian chant! I'll be checking out that YouTube account to get the skinny, I promise.
Here's the (Gregorian) Communio, with the score embedded in the video:
(I must add that, strangely enough, our reading from John leaves "I am the Good Shepherd" out entirely! We read only John:1-10, but the Good Shepherd parts of the text are in verses 14 (the Communion Song) and 11 (the Alleluia above) (see all of John 10 here). Very odd! But, we do get these verses (11-16) in Year B, so we'll hear them eventually.)
And how about this, for Good Shepherd Sunday? Wikipedia says it's from around 300 CE; that it's marble, 92cm high, and that the legs have been restored. The sculpture resides in the Vatican Museum in Rome.