Installing PHT on Ubuntu 15.04 wasn’t entirely straightforward, and required a bit of Googling. I don’t know if all these steps are necessary, it is just the order in which I found and tried helpful hints from a variety of forums.
sudo add-apt-repository ppa:plexapp/plexht
sudo apt-add-repository ppa:pulse-eight/libcec
sudo apt-get update
sudo apt-get install plexhometheater
sudo apt-get install libcec-dev
sudo apt-get install mkvtoolnix
sudo apt-get install ffmpeg
At this point PHT was installed and launched, but all videos had no sound and were playing at 2x the normal rate. I did two things to resolve those issues.
1. Uncheck Dolby Digital AC3 capable receiver under the Plex audio preferences
2. In Ubuntu, System Settings, Sound: made sure the HDMI was set correctly and tested the sound.
3. Make sure all the suspend/sleep/screen lock settings are turned off. ‘System Settings’ ‘Brightness and Lock’. Nothing worse than the PC suspending mid show:)