Sorry, I hate to contradict, but this is incorrect. Decay times do not increase as volume increases.Yes they do. As we increase the volume. We load the room with more sound energy. So the decay times start increasing with volume. So it is imperative, that room measurements are made at the usual listening volume. As at any other volume, either less or more than this volume, the decay times across the frequency range will change.
The only significant volume related effects are those associated with how human hearing response varies (Fletcher-Munson curves).