AMD's DC code landed in kernel 4.15, bringing HDMI/DisplayPort audio to the AMDGPU driver.
Though this is enabled for Vega GPUs in the current 4.15.3-300 Fedora 27 kernel, it isn't enabled for pre-Vega cards:
$ grep AMD_DC /boot/config-4.15.3-300.fc27.x86_64
CONFIG_DRM_AMD_DC=y
# CONFIG_DRM_AMD_DC_PRE_VEGA is not set
This means audio over HDMI/DisplayPort is not available for GCN 1.2 to GCN 1.4 cards (or GCN 1.0/GCN 1.1 if using the AMDGPU driver).
Setting the following would fix this:
CONFIG_DRM_AMD_DC_PRE_VEGA=1
This is already being used by Arch by default for example: https://wiki.archlinux.org/index.php/AMDGPU#Enable_AMD_DC_on_pre-Vega_cards
I have tested AMDGPU+DC with GCN 1.1 hardware with the following kernel parameters, and can confirm that it results in working HDMI audio (tested with 5.1) and DisplayPort audio (tested with stereo) on my hardware:
amdgpu.cik_support=1 radeon.cik_support=0 amdgpu.dc=1
In my case, the AMDGPU driver doesn't exhibit a long-standing bug present in the Radeon driver; I suspect there will be other users who would also benefit from having a fully enabled (with sound) AMDGPU driver for pre-Vega hardware.
*********** MASS BUG UPDATE **************
We apologize for the inconvenience. There are a large number of bugs to go through and several of them have gone stale. Due to this, we are doing a mass bug update across all of the Fedora 27 kernel bugs.
Fedora 27 has now been rebased to 4.17.7-100.fc27. Please test this kernel update (or newer) and let us know if you issue has been resolved or if it is still present with the newer kernel.
If you have moved on to Fedora 28, and are still experiencing this issue, please change the version to Fedora 28.
If you experience different issues, please open a new bug report for those.