What is the reason of sound from the attached file? From time to time I hear this sound without any visible notification: https://drive.google.com/file/d/0BwUf1IftWc6YUk1BWDFoQ1A2SW8/view

What is the reason of it?

Usually I have the following apps opened: Chrome, Skype, Telegram, Sublime Text, IntelliJ, iTerm, Commander One, Postman.
macOS 10.12.3
James P, Thanks for the hint! It was Skype indeed.

To turn off this sound I went to Skype -> Preferences -> Notifications -> Overview and unchecked "Play sounds related to contact availability"