Mr. Pearson, thank you very much for your reply with suggestion.
My speakers are powered from AC 120 wall outlet. I may take your advise to buy Amazon.com: TanTan Smart Plug WiFi Wireless Mini Socket Outlet and use it conjunction with Air Fly mouse and Key pad remote controller unless someone knows some appropriately-crafted software, the NUC could tell the socket to drop power to the speakers so that I am able to use one key stroke instead of 2 key strokes on air Fly Remote controller to turn off the system.
While we are on this subject, if my speakers are powered by USB then I have to change the BIOS setting. The NUC7i7BNKG BIOS for USB setting indicates S4/S5 together. Is this means whatever I use on S4 also loose power if I set not to power S5?