So I am back with Ubuntu 14.04 LTS for the moment because I figured out what was wrong with audio all this time. (multiple things)
can’t wait for the next ubuntu LTS or will go back to Fedora, I swear… the lack of proprietary driver support is the only thing I dislike there. the rest is awesome.

1) Pulseaudio is just shit without a proper systemd implementation (works like a charm for fedora 23)
2) HDMI passthrough has to be specifically enabled in plex settings
3) Nvidia prorietary driver needs to be used for proper hdmi audio passthrough
4) pulseaudio fix needs to be uncommented in /usr/bin/plexhometheater.sh to disable it
5) alternatively apt-get remove pulseaudio < -- after setup, will try again in April 2016 6) HDMI passthrough on your amplifier is counter-productive / sets itself to the wrong source when you sleep the media center / wake without amplifier enabled 7) HDMI-CEC needs more work, now it is just fuckign things up when enabled so DISABLE for now (once I have playstation 4, chromecast and more HDMI stuffz I can re-enable it. after a lot of testing and playing around with it I can now watch AAC, DTS and Dolby surround 2, 5 and 7 channels - everything works and survives reboots.

documenting what fixed it for me – I wanted a plex server that also acts as player and steam box
so:

install ubuntu 14.04 lts
set it all up – add plugins-bad and -ugly, add proprietary extras
install third-party GPU driver

steam works out of the box, so does the xbox360 controller
IR controller for plex.. who knows a good one?

install these repositories and then install plexmediaserver via plex.tv downloads and plexhometheater from here:
https://forums.plex.tv/discussion/87253/plex-home-theater-linux-builds

add-apt-repository ppa:pulse-eight/libcec < -- only if you want that. I run everything via my amplifier so no need for that add-apt-repository ppa:team-xbmc:xbmc-nightly add-apt-repository ppa:plexapp/plexht logitech keyboard works with solaar this guy: http://www.jorgecastro.org/2014/08/21/the-ubuntu-steam-box/ more or less does what I want so I base my build on his website currently testing with ubuntu 14.04.02 but found out latest xorg is breaking this one first thing: install all updates including non-free and partner repos
enable the openssh-server (adjust iptables/firewall/whatever you run and set up fail2ban, rkhunter and logwatch)