So far, outside of some software crashes, I love the Zwift product. The metrics are right on with my Garmin and I run Trainerroad program at the same time.
I would add a volume control as the sound dominates over my streaming music. I would also add, in the future, a notice when KOM or Sprint section are coming up.
As a work around for volume control, you could try changing the volume levels in the task bar volume mixer (Just right click your speaker icon in the task bar and adjust the sliders to your preference)