The lyrics are verbatim from the encyclical, and they are as follows:
Holy Mary, Mother of God,
you have given the world its true light,
Jesus, your Son – the Son of God.
You abandoned yourself completely
to God's call
and thus became a wellspring
of the goodness which flows forth from him.
Show us Jesus. Lead us to him.
Teach us to know and love him,
so that we too can become
capable of true love
and be fountains of living water
in the midst of a thirsting world.
The song in mp3 format is free for you to download.