100v,115v versus 230v

Audio gear such as amps, preamps, tuners, CD players etc typically have transformers inside. Whatever the line in voltage is, that line in voltage is changed to whatever the component actually runs on 

"To add further insult to injury, the amount of time your pump runs to circulate the water will also double (compared to a pump in a 220-volt system)."     WRONG!   The typical, dual voltage, AC(single phase) electric motor, when attached to a pump, runs at the exact same RPM and HP (https://spicerparts.com/calculators/horsepower-torque-calculator), wired for either voltage.      The only thing that will change, is the current draw, which will drop by half, at the higher voltage.      ie: https://www.youtube.com/watch?reload=9&v=3llc68VvzsY     Notice here, that though the motors are all dual voltage; only one RPM is mentioned for all and one HP rating for each, in the specs: https://www.poolblu.com/Public/pbsShowFamily.aspx?xFM=FM043298&xCT=

Some of my  japanese friends who they live different part of japan where voltage is 200 volt they say their audio gear it sounds better then 100 volt supplied areas.Known fact all the audio gear has transformers except OTL Amps etc, another person I know who he Returned from USA brought his  120V ARC REF 610 Power amps to UK He is saying that he is getting better results using step up/ down device.

Sounds like an Ask Paul episode.
He could easily compare his own stuff with a Power Plant and identical amps set to Schuko.

Okay, real answer: safety, economy, and practicality. The safety aspect is its simply much harder to shock yourself with 120, and when you do its no big. The economy aspect, higher voltages require less wire for the same power. This is why transmission lines are all high voltage, they allow a great deal of power to be transmitted a long way using relatively thin wire. The difference between 120 and 240 is less, but still if you multiply by hundreds of millions of homes its a lot of copper.

That leaves us with practicality. In Europe, and areas without a lot of copper it was more practical to balance the risk/cost towards higher voltage. In the US copper was plentiful, we could afford to be safer.  

The US system is by the way 240V. High voltage transmission lines feed power to transfer stations where huge step down transformers lower the voltage for distribution to smaller areas. Up on telephone poles or sometimes on the ground in big boxes are more step down transformers that lower the voltage again, this time because its going to your home its stepped down to 240V.  

This last step-down transformer has three taps. Two are from different windings on the transformer. The third completes the circuit by grounding back to the transformer core. Thus between two of the taps is 120V, while across the whole transformer its 240V.

The three wires that bring service to the home are the neutral wire and the two 240V wires. The two rows of breakers in the panel are the two hot wires bringing 240V into the panel. They are attached to two copper bus bars running the length of the panel. The two rows of breakers attach one to each bar with the circuit being completed by going back to the neutral bar running along one side of the panel. This is why a 240V breaker takes up two slots- it connects across to both of the bars. 120+120=240.  

The whole thing is quite safe, as you will discover if you ever actually work with the thing. It really is quite hard to shock yourself with 120V, and a whole lot easier with 240V! 

But I know all this accurate truth is gonna blow a fuse, so don't take my word for it. This video was made to reach even the H18. Well, some of em anyway. https://www.youtube.com/watch?v=jMmUoZh3Hq4

This is my system which is wired 240V to a step down transformer so only the last five feet or so is 120V. https://systems.audiogon.com/systems/8367

Anyway, that's the answer. The US really is 240V, but use mostly 120V inside the home because its safer. Please do watch the video. It makes this all crystal clear. Almost as clear as my prose. But delivered with a lot more energy.