Very often, when you hear music or go away, your machine will first idle for a few minutes (whatever your screensaver is set to), then go to screen-saver mode and in case you have that set, after a few minutes your screen / LCD / monitor will completely shutdown, mostly first the LCD backlight, followed by the screen actually switching itself off. But it doesn have to be that way: you can turn off your screen immediately. On Ubuntu, you probably have DPMS (Display Power Management Signaling) working, and you can force it to shutdown the screen immediately:
xset dpms force off
This is especially handy if you want to save battery on a laptop, or when you need/want the display to go completely dark immediately.
The manual way to create a launcher is pretty well described here.
Create a file with these contents:
Comment[en_EN]=switch off / turn off screen
Exec=xset dpms force off
Comment=switch off / turn off screen
or do this: $ gnome-desktop-item-edit ~/Desktop/xoff.desktop --create-new
Add the executable bit: $ sudo chmod +x ~/Desktop/xoff.desktop
And in case you've created the launcher in the .desktop file directory /home/<username>/.local/share/applications/,
update the Unity / launcher database with: $ sudo updatedb