Many vocal groups, particularly vocal jazz groups, place a high value on the ability of each singer to sing her part as a solo. Why?
The reason is that vocal jazz is incredibly challenging both vertically (understanding how your note fits with the rest of the chord) and horizontally (making sense of a chromatic, winding part).
Have you ever had it happen that you hear a song title and realize you don’t know the melody – you only know your part from when you learned it in choir? What that means is that you learned your part well enough to sing it as if it’s a melody. That’s no great challenge in much classical choral music: well-written choral parts tend to feel fairly melodic. Unfortunately, that’s often not the case in the harmonically sophisticated writing in great vocal jazz.
The sad truth is that your singers will never be able to consistently sing this music well until they can sing any line as a solo. Any part of any song…get the first note and go. It’s straightforward to do that with, say, the alto part on the Vivaldi Gloria, but given a section of Phil Mattson’s Body & Soul and you’ll be in for a much longer and more difficult challenge. It does come, though. With practice, strange-sounding chromatic lines, diminished arpeggios, and unwieldy leaps can come to feel as natural as Twinkle, Twinkle, Little Star. And that’s when your choir comes alive.
It’s a big challenge, but it’s the right challenge–and it’s the only way to effectively sing challenging music well.