I had previously tried changing real-time audio volume, but I bet I overlooked enabling/disabling the ‘real-time’ setting. It had previously been working in 4.9 so it’s possible I inadvertently disabled ‘real-time’ somehow.
I had to delete my dev branch and resync from Perforce for other reasons, but I’ve verified that the real-time audio volume was turned up and ‘real-time’ was enabled, and audio preview is working again.