It's Pronounced "EEEN"

As Christmas carols start filling the air, my heart fills with joy.

And angst.

That is, there's one thing about Christmas carols among English-speakers which always bothers me. (Because the issue was so deeply impressed upon me by my beloved high school choir director. And if it mattered to her, it matters to me!)
And although I doubt I'll ever make a big enough impression to change the way the English-speaking world sings Angels We Have Heard on High if I reach just one or two or maybe twenty people, well, that will be time well-spent.
So, here's the deal:
"Gloria in excelsis Deo" is a Latin phrase. As in, all four words in the phrase are from the Latin language. Although "in" also looks like an English word, in this phrase it is Latin - and therefore should be pronounced as a Latin word.
Do you know how to pronounce "gloria"? Go ahead and say it out loud.
Did you notice the sound the "i" made when you said "gloria"?
Now try "excelsis".
Did the "i" make an "eee" sound again?

Fun fact: The "i" in the Latin word "in" makes the same "eee" sound as the "i" in the Latin words "gloria" and "excelsis".

So, give the Latin version of "in" a try.
Go ahead. It's pronounced "eeen".
By the way, you'll help convey the accurate message if you utilize the correct pronounciation, because Latin "in" means "to" (Glory to God in the highest!). If a Latin-speaker hears you throw in the English "in" while you're singing the Latin phrase, they might get confused.

OK. I've made my case. Thanks for listening.
And, please - for the love of Latin - tell your caroling friends that it's pronounced "eeen"!
*wink*