The sound comparison method, in my opinion, would prove nothing except possibly a severe mechanical problem. There are no published specifications for “normal” engine sound.
Later models of the NC700X and NC750X call for valve adjustments at 16,000 mile intervals, vs the earlier 8000 mile recommended interval. Some of us have decided on our own to make that new schedule retroactive for earlier model years. I think Honda was being conservative with the early specs.
My advice would be to check the valves now and then every 16,000 miles thereafter, subject to changes based on observed necessity for actual adjustment. It’s the only way you’ll know for sure.
In general, I have no reason or desire to service the engine any sooner than Honda’s recommended intervals unless there is evidence of a problem.