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

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.