The carol Adeste Fideles includes the line "Cantet nunc io chorus angelorum". What does "io" mean?
Iō is an interjection, defined in Lewis & Short as expressing joy, like English "Hurrah!"; or pain, like English "Oh!"; or hurriedly calling to someone, like English "Come quick!"
Some books with the words to Adeste Fideles, like this one, put io in quotes like this:
Frederick Oakeley's English translation renders the first two lines:
Sing, choirs of angels, sing in exultation;
A literal translation would be:
May the choir of angels now sing "Io!"
May the choir of angels now sing "Hurrah!"
It looks like the ritualistic exclamation, much like in io Saturnalia, the line is translated as "Sing choir of angels" but the Latin says "Now let the chorus of angels sing" so it seems to be little more than filler.
The only other explanation (and this would be a stretch) is that it is a weird spelling of eo dative of is/ea/id, i.e. sing to him but I highly doubt that, especially since it would be ii, since eo is the ablative.
So I'd take it as the Latin equivalent of the "O" in O come all ye faithful, doesn't really have any consequence on the words, and is little more than a la or oh or something similar.
Also, looking in Lewis and Short's we have miles, io, magna voce, triumphe, canet so it seems to be used with cano/canto.