Unto us a Child is Born! is a bright, rhythmical version of an old chant-like tune, Puer Nobis Nascitur, from a collection published in 1582 called Piae Cantiones (from which we find find carols like In Dulci Jubilo). I have played with the rhythms, the words and the harmonization. I think it turned out rather joyous:

• Unto us a Child is Born! : free sheet music pdf
• Unto us a Child is Born! : midi realization mp3

You can find an arrangement with an alternate second verse on ChoirWorks.com.

I wasn’t quite happy with translations I found here, here and here. So, I borrowed, adapted and added. Here is my collage of the text:

Unto us a Child is born! Comes to earth a stranger;
Ox and lamb their owner know: he’s cradled in th manger.

Of His love and mercy mild, this the Christmas story:
Mary’s meek and gentle child will lead us up to glory!

Alpha and Omega, Thine be this day our chorus;
King of kings, and Lord of lords, now let Thy grace be o’er us.