Bought the game just now, and realized this issue on my first mouse click.
This is an extremely high-pitched noise. I'm not surprised that many people are not bothered by it. As someone who is sensitive to high pitched noises it makes the game completely unplayable with the UI sound on. It's not just annoying, but literally painful.
Because the issue is the pitch of the noise, not the volume, the sound have to be turned down below audible volume to stop it from being annoying and painful. Just lowering the UI volume a bit is does nothing at all unfortunately.