sound princess; toilet sound device
A trademarked device installed in women's restrooms in Japan that plays a flushing sound or melody to mask embarrassing noises. Often activated by a button or motion sensor.
When I entered the restroom, the sound princess automatically played the sound of running water.
When you press the sound princess button, you hear a sound like a babbling brook.
From 音 (おと, 'sound') + 姫 (ひめ, 'princess'), a playful name suggesting a princess who provides a pleasant sound. The exact origin as a product name is a trademark.