The retort conveniently fails to mention Nikhil's suggestion that reality is "reduced to math" and that it is possible for many or infinite spacetimes to exist. This can be supported by the fact that scientists have discovered that sound waves do create patterns in sand, meaning that though the wave may still exist, its shape is dictated by the observer's perception. This supports the idea that the perception of reality is what enables it to exist.
Furthermore, the retort does not consider the importance of life in sustaining our universe. It may be true that sound exists as a wave regardless of whether or not there is a listener present, but if there were no life, then our universe would still disappear for all practical purposes. Life capable of perceiving the waves is what makes reality an observable experience and without it, we would be unable to experience our true universe.
So while the retort is accurate in that sound is a wave, it fails to consider Nikhil's argument holistically, overlooks the importance of life in allowing us to perceive reality and also leaves out key concepts in Nikhil's essay such as his reduction of reality to math and the possibility of infinite spacetimes existing. Therefore, UrRong! Nikhil's conclusion holds true: a universe without intelligent life to observe it wouldn't really exist at all—for all practical purposes, at least.