I went into dxdiag for directx and moved the acceleration for the sound to the extreme right.
I had moved the acceleration slider to none after reading a suggestion for no voice problem, and had left it there even though it made no difference.
Now have traffic at 20% commercial and private.
Maybe it's those settings, or maybe it's just a different day, but ATC is now being polite enough to allow me to finish my sentences.
Not exactly hard evidence, but it's working as I would expect now.
Anyhow, problem solved or it just went away on its own...
Thanks for the suggestions, Sunny.