https://bugs.freedesktop.org/show_bug.cgi?id=77677
--- Comment #31 from Garrett socalfisher@gmail.com --- (In reply to comment #23)
This might be a duplicate of bug 76837. Does disabling dpm help? Append radeon.dpm=0 to the kernel command line in grub.
Adding radeon.dpm=0 to boot params did not work. I am sorry to report. On my 7750 card. HDMI sound is machine gun like, still in Openlec.