HDMI connection not recognized on Laptop with Ubuntu 14.04

On a Laptop with Ubuntu 14.04 I am not able to display the current screen's stuff on a Monitor connected with HDMI. The HDMI cables are both plugged in to the laptop and the monitor, and the monitor is switched on.

Going to

Systems Settings -> Displays

and clicking on 'Detect Displays' only the standard laptop screen is shown. The external monitor is not shown.

How can I fix this problem in order to see the current screen on both, the laptop screen and on the monitor screen?

Also, it is unimportant if the screen can play the laptop's sound. I only want the visible screen output shown on the external monitor...

Additional information:

• xrandr only shows the standard monitor


• The HDMI connection works flawless when running the laptop with Windows (dual-boot)