"Rejoice! Rejoice! Emmanuel
Shall come to thee, O Israel!"
Most people understandably tend to breathe after the word "Emmanuel." It's the end of a line, right? And in most musical settings the last syllable of the word occurs on a longer, held note. It's a natural place to breathe. The problem is that when one breathes there, the sense of the line is confused and it sounds like we are telling Emmanuel to rejoice instead of telling Israel to rejoice because Emmanuel is coming.
For this reason, I prefer the musical setting of this hymn in which the "-el" of "Emmanuel" is given only an eighth note and is followed by a series of eighth notes that carry through to the final syllable of the final word. Keeping the rhythm moving more effectively preserves the meaning of the line. Then a quick breath can be taken at the comma after the word "thee." A skillful lead musician will make sure the accompaniment supports this approach, allowing time for the singers to breathe at the comma.
The likelihood, though, is that this Advent you and I will sing the setting of "O Come, O Come, Emmanuel" that has a long note at the end of the first line. In that case, I encourage you to do what I do. Take a nice, deep breath after the second "Rejoice" and try to sing all the way to "thee" on that one breath. After "thee" take a quick catch breath to finish the refrain. You may be the only one holding out that "el" but you can pat yourself on the back for doing your part to preserve the poetry of the hymn. And who knows, maybe you will start a new trend!