Hot answers tagged hdmi
The 3.19.0-18 kernel has a bug that breaks HDMI sound on intel HDA. Rollback your kernel by running sudo apt-get purge linux-image-3.19.0-18-generic Note this will also remove linux-generic* and linux-image-generic* which means Ubuntu will never offer to upgrade your kernel ever again, so you need to remember to check if it's fixed in a new kernel in a ...
The issue appears to be lack of MST support in the Linux kernel. Upgrading to Ubuntu 15.04 with kernel 3.19 fixed the issue for me, but only after I installed the proprietary nvidia drivers.
It looks like you're trying to use three monitors on one card which, as far as I know, isn't possible. Like you're experiencing, the third monitor will be recognized, but trying to enable it either won't work, or it will disable the non-main (secondary) monitor. It's not a driver problem or a problem with Ubuntu, it's just not possible.
Only top voted, non community-wiki answers of a minimum length are eligible