This could be a big subject – well beyond my experience and knowledge. I confine myself to a simple observation.
The other day I found myself singing this – the opening of the Art of Fugue
This was not completely random. It had been on the radio that morning. I just thought how delightfully singable it was in
contrast to some of Bach’s vocal lines which feel very instrumental. There can be nowhere to breath, they can be virtuosic and difficult to sing for reasons of speed or melodic shape. Here’s the first example that came to mind.
In spite of what I’ve said, just looking at it makes me want to listen to it or sing it (with orchestra and choir)