If you change the - actually 2 6th notes to a quarter note, isn't it?

No, it's two eighth notes

- you'd have the right ratio of notes per syllable for the last verse only. Of course, you could always change "voices" to "voice" in every verse.

You could put "--" in for the missing syllable.

Or, just treat the last line as a coda, and have it different for that melody line only (Which is what I've done, with NWC, just to see how it sounds. Sounds okay!

Now...
There's nothing wrong with using the same last line throughout the song. My guess is that if you sing this around a a bunch of singers, they'll pick up the last line and sing along. If you change the verse, you're going to have to give people hand signals or wiggle your eyebrows at them to let them know they have to be quiet for the last line because it's different.

So I favor using "So raise up your voices. Fill the air with sweet song!" as the last line of every verse.

Yes, but then it won't rhyme. Of course, there's no law that says it has to rhyme, but there's been at least a near rhyme all the way through, and a sharp left turn at the end, may make people double-take even more violently than share/air. So now, I thinking about changing the penultamite line:

Lessee: "She lives in our hearts, and her love makes us strong" scans okay, but that would mean using the strong/song rhyme twice in a row... Again, no law against that, but I'd prefer not to. How about: "She bides in our hearts, with us all the year long"?

Just a thought...

It really is easier to change the words than reposting all the midis!

...but YMMV