This is why I love Plex.
I thought I would share my learnings as after a couple of days of searching I was unable to find a single clear source of information.
I wanted to replace my Sonos speakers with a device that used RasPlex as I found the Sonos expensive and inappropriate solution. Everyone already knows how to use Plex in the house and we have it everywhere via RasPlex.
I used the RasberryPi with RasPlex (http://www.rasplex.com/) solution combined with a HiBerry+amp (https://www.hifiberry.com/products/ampplus/) as well as a few second have Bose cube speakers to build a very cool solution very quickly.
The only part that was not entirely obvious was managing the volume control for the HiBerry. This turned out to be super easy given the great Plex client available and the quality of the RasPlex.
After the bits arrived it all arrived it took less than 20 minutes including the problem resolution.
Step 0.
Install RasPlex on the Sim as described on the site.
Attach the HiBerry to the RasberryPi
Put in the SD card, attach the screen, a keyboard and power so you can complete the final configuration.
I changed a few setting like autologin, connecting to the wireless network etc.
Don’t forget to change the Unit name to something meaningful.
Step 1.
Once installed edit the config.txt document in the root of your RasPlex This can be done directly from windows.
Comment out by adding a “#” at the start the dtoverlay line as shown below.
# Required overlay for LIRC to function
# dtoverlay=lirc-rpi
And uncomment or remove the “#” from the start of the dtoverlay line you will see a few lines below.
dtoverlay=hifiberry-amp
Step 2.
I used this referance: https://support.hifiberry.com/hc/en-us/articles/205385241-Mixer-controls-on-the-HiFiBerry-boards
run the following command to set the default volume lower. If you test it before you have done this expect it to be LOUD.
amixer sset Master 70%
also run teh followign to staore your setting.
amixer sset Master 70%
I found I got an error and had to create the final directory.
Step 3.
Grab my phone, choose the new RasPlex player from the player list, hit your favourite song and go.
The phone volume controls will control the volume.
Step 4
Grab a low profile electronics box metal, paint it matt black, put your new headless audio player in it, attach your choice of fancy speaker.
Don’t forget the “Better than Sonos” logo 
Meta-data: Plex, Rasplex, HiBerry+AMP, Volume control, headless audio
