how to connect hdmi on xps13

When I run xrandr I can see that HDMI1 is disconnected but how do I connect HDMI? In fact I can get the picture on my TV but no sound.

I have tried with the oem-audio-dkms driver and also with the native kernel 3.19 drive with the same results.

With the factory installation I had both picture and sound but when I installed kernel 3.19 the sound went off.