Open a Konsole (or whatever that terminal screen with a black background and all characters is called in your version) and type alsamixer
You'll see a number of VU meters, most probably your headphone output is either 0 or muted. Unmute with space bar, change volume with arrow keys.
If alsamixer is not installed, do sudo apt-get update && sudo apt-get install alsa-utils
jlinkels
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