I love bells and whistles while using an OS and clearly there's scarcity of sound themes in Linux so I made one myself which turned out quite good, I believe. I tried using sounds that would not be annoying and would still be pleasant to hear after a couple of months of using so I called it "Smooth".
You can get it here:
To install it just extract and place the folder "Smooth" inside ~/.local/share/sounds/ (you might have to create the "sounds" folder). Here's a printscreen:
To set it just go to sound settings (Control Center > Sound) and at "Sound Effects" choose "Smooth" for the Sound theme, Don't forget to have "Enable window and button sounds" checked: