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Of course you can't go more than half a bar in arranging a cappella music before you find a note in the melody that doesn't belong to the prevailing harmony, so in some ways this post is about a central and obvious thing arrangers are always looking at anyway. But a couple of projects earlier this year (songs from Sondheim and the Beatles) have got me thinking about this specifically in terms of how questions of timbre affect our choices.

Non-chord tones in a vocal melody floating across a backwash of instrumental accompaniment have a whole different sonic effect from those same notes sung in a texture where the accompanying harmonies are of the same type of sound as the melody. Sung accompaniments pull the vocal non-chord tones into the chord where instrumental backing lets them stand apart. In a cappella textures, you are much more likely to find a melodic note infecting the harmony, changing its colour.

So, what strategies do we have to deal with this?

1. The traditional barbershop approach is to find a chord that will accommodate the non-chord tone and use that instead for that one note. Indeed, most of the outlying chords in the barbershop chord vocabulary - the ones with all kinds of rules hedged about their usage, and that that non-barbershoppers are astonished to hear classified as 'consonant' (major 7ths, added 9ths, augmented dominant 7ths) - are actually instances where you can stick a non-chord tone across a chord and still make it work without getting distracted from the primary harmony. The ear accepts a major 7th as a melodic passing note, but sustain it in an accompanying part and you have a very different harmonic flavour.

   If you're working with a song that mostly uses traditional barbershop harmonic language, this works well. But songs that are more predominantly triadic (some bits of the Beatles) or that use more extended, layered harmonies (Sondheim) don't always respond well to this, so other strategies come into play.

2. Mess with the texture. In the original versions, it is the timbral distance between melody and accompaniment that makes sense of the non-chord tones. A cappella genres use a more homogenous timbral palette, but you can still open up perceptual distance between melody and accompaniment through texture. Homophony, at these moments, is not your friend; go for the vocables.
3. Mess with the voicings. A chord that sounds hokey voiced closely may sound fine with a bit more space in it; or a different inversion might fix the problem; or, in more complex harmonic worlds, deciding to omit a different note gives the ear space to untangle things when melody and harmony aren't playing together nicely. I have yet to develop systematic solutions for this - I'm still doing it by a combination of intuition and trial-and-error - but I would say that it pays to be careful with leading-notes (and functional leading notes), which can box you in unhelpfully.
4. Change the melody note. This is of course the one that barbershoppers get very shocked about, with their rather literal texttreue approach to the concept of musical works, which often strikes me as having a greater affinity with the classical tradition than the popular traditions they use as source material. (More on this, if you’re interested, here.) And it’s not something I’d recommend as a first resort (which is why it comes a way down this list). But just occasionally it offers a solution that sounds less hokey than all the other possibilities available.

   I’m not advocating relinquishing our responsibility as arrangers to the composer here, nor even balancing it with our obligation not to make singers sound stupid (which is, however, arguably more important). What I am pointing out is that harmony is also part of the song, and sometimes, when melody and harmony are pulling in different directions, when you can’t include both the chord and the tune without a vile dissonance, you have to pick between them. And there are places where a substitute harmony will work, and there are places where you really need the original chord.

The other option, of course, is to pick easier songs to arrange, but when people want to sing stuff, it’s nice to be able to make it possible for them.