It is considered a kind of rude in Japan that a female makes any sound when she is using a toilet. Japanese girls may flush the toilet beforehand in order to cover the sound, but it is both inconvenience and a waste of water. Pee undercovered is the answer to the problem. This app shows a toilet flush tank and, when the flush button tapped, it makes different kinds of sounds such that you can act "undercovered".
Tip: listening to the waterfall sound is also a kind of joy even if you're not using the toilet. enjoy!