This is a solution or a work around to getting audio functionality via the Display Ports on the HP Elite 8300 Ultra Slim.
Download the HP Power Assistant Software v2.0.5.1
http://h20565.www2.hp.com/hpsc/swd/public/detail?swItemId=ob_102595_1
and install it and suddenly in the Sound settings in the Windows Control Panel you will find it now shows audio devices that are connected via the Display Ports via HDMI 
, weird I know but it works.
HP Customer Services told me categorically that it was not a function of the 8300 Ultra Slim supports due to the revision of dispaly port it has so I am a bit annoyed that I had to search and search and acutally fix it myself.
