From the Delerium Wiki:

The Mediæval Bæbes provided the vocal track for, and starred in the video of, Aria; the vocals are an adapted version of the vocals from "All Turns to Yesterday" on the Bæbes' Worldes Blysse album. They are also featured on two tracks from Delerium's 2006 album, Nuages du Monde.

'Middle-English' Lyrics From the youTube poster.

Lyrics:
I have wist, sin I couthe meen,
That children hath by candle light
Her shadewe on the wal iseen,
And ronne thereafter all the night.

Bisy aboute they han ben
To catchen it with all here might,
And whon they catchen
It best wolde wene,
Sonnest it shet out of her sight.

The shadowe catchen they ne might,
Fo no lines that they couthe lay.
This shadewe I may liken aright
To this world and yesterday.

Translation:
Ever since I could remember, I have known
That children, by candle light,
Have seen a shadow on the wall
And have chased it all night through

Busily, they have tried
With all their might, to catch it.
And when they most
Expect to catch it
It shoots most quickly out of sight.

This shadow never shall be caught
In any trap they lay.
This shadow in the likeness,
Of this world and yesterday.

---

Tag note, Delerium are Canadian, and the Mediæval Bæbes are English. If any of them come across this and are single, let me know ;)