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Show messagebox when session or break ends.
It will be better if a message box pop up on the screen when a session (or timer) ends. It should prompt user to start a new session. And this should be configurable. Not all user will be willing to get a message box.
Playing sounds down not bring attention to it all the times. Many times while working in the office, we do not have sound cards enabled or keep it muted. A message box will be a better option.