It's spelled with only one 'U' -- Tantum Ergo --- even though it is pronounced like "taunt" in English. Latin 'a' is broad.
We sing it at Benediction of the Blessed Sacrament, a devotion of the Roman Catholic Church. It can also be spoken, but I think that's rather uncommon. Not that I'm an expert.
Here's a Wikipedia article about it: http://en.wikipedia.org/wiki/Benediction_of_the_Blessed_Sacrament
However, Wikipedia is NOT authoritative regarding Catholic doctrine. I cite this particular article because it has a lot of photos, and the information looks pretty accurate.
Benediction is a Catholic devotion, not really a matter of doctrine.
For the authoritative information on the Roman Catholic Church doctrine or dogma, you would go to the Vatican's own English website, or the United States Conference of Catholic Bishops --- usccb.org.
There are many Catholic resources online. EWTN's website is a good place to start, also. They have televised Benediction on EWTN. You can also find You Tube videos of Benediction, with or without Tantum Ergo.