Skype plays a bubbly sound to alert me to something. The tool bar icon also bounces. I use MAC. When I go to Skype and look to see if I've been messaged or if someone is trying to connect, there's nothing. What is that sound?
You might open Skype preferences >>> Notifications >>> Details.There you probably will be able to know what event refers to the bubbly sound.
And untick if needed.
Check Preferences > Notifications > Details
and look for the event "when contact becomes available"
I had this turned on (it's on by default), so every time someone in my contacts list changed their status to "available", I got a notification.
Turning off the notification for this event solved the problem.