If as you say you are connecting to the Line-in jack, then this is the wrong place to put your headphones
. Line in is for external devices, mp3 players etc.
You should be connecting your headphones into the Headphone/speaker/line-out jack
This is a list of the connections on your model ;
4x USB 2.0 ports
1x external display (VGA) port
1x Headphone/speaker/line-out jack
1x Microphone-in jack
1x Line-in jack
1x Ethernet (RJ-45) port
1x HDMI™ (High-Definition Multimedia Interface) port with HDCP (High-bandwidth Digital Content Protection) support
1x Modem (RJ-11) port
1x DC-in jack for AC adapter
Please Post back on your progress. It’s nice to know if the forum has helped or not.