Unable to change sound Output when headphone pluged in

MacMini 2012 ML, conected by HDMI to Samsung Monitor with speakers.

As show in pic1, I can easilly select between the Macmini internal spekar and the Monitor Speakers

If I plug in a headphone, its impossible to select the Monitor speakers. The option is visible, but when I click on it, it automatically goes back to headphones. (it even makes a bouncing effect, what the?)