Have the same issue with Radeon HD 6850 and 15.10 beta. Got the same problem with earlier drivers.
Have strictly the same behavior in different combinations of my connected DP and HDMI devices.
If HDMI connected boottime - audio is always driven to it until reboot (and, then, if HDMI is disconnected then, it will be on Display Port), but I need it go to Display Port even if HDMI is connected without disconnecting HDMI every boottime!
Seems that it's the well-known problem - look at reddit thread https://www.reddit.com/r/buildapc/comments/2hl6it/problem_with_video_card_choosing_wrong_monitor_as/
Also here: The specified item was not found.
The problem source is either card BIOS logic determining to activate audio on HDMI preferred, or AMD Catalyst drivers logic, determining HDMI audio preferred during boottime.
So, need to BIOS logic be changed or AMD Catalyst drivers should got feature of software audio driver switching or preferred driver setup between Display Port and HDMI interfaces.
How could this be done?