Can't get HDMI audio to work for ATI HD 5450 with default radeon driver despite enabling radeon.audio=1

I've been trying to get HDMI audio working for my ATI HD 5450 using the default "radeon" kernel drivers.

I have a vanilla install of RHEL except for getting the latest updates from yum (and have also tried Fedora 23, Ubuntu 16.04 and 17.04, and possibly others I can't remember).

I've been scouring the net for an answer but no luck yet.

This article provided some useful info but I still can't get any sound: http://forums.fedoraforum.org/showthread.php?p=1786436#post1786436

I've added the "radeon.audio=1" argument to the GRUB2 config with grubby.

sudo grubby --args="radeon.audio=1" --update-kernel /boot/vmlinuz-3.10.0-514.el7.x86_64

Confirmed with grubby that it's been applied:

$ sudo grubby --info=ALL
index=0
kernel=/boot/vmlinuz-3.10.0-514.el7.x86_64
args="ro crashkernel=auto rd.lvm.lv=linux/rhel-root rd.lvm.lv=linux/swap rhgb quiet LANG=en_AU.UTF-8 radeon.audio=1"

It has taken effect in /boot/grub2/grub.cfg:

        linux16 /vmlinuz-3.10.0-514.el7.x86_64 root=/dev/mapper/linux-rhel--root ro crashkernel=auto rd.lvm.lv=linux/rhel-root rd.lvm.lv=linux/swap rhgb quiet LANG=en_AU.UTF-8 radeon.audio=1

I restarted and can see with modprobe that the argument has been applied to the radeon driver:

$ sudo modprobe -c radeon | grep -i audio
options radeon audio=1
options radeon audio=1

However still no audio. No sound from speaker-test. No sound from CD. No sound from the GNOME speaker test in System Preferences.

I installed pavucontrol but nothing is muted. I tried stereo and 5.1 but no improvement.

I checked the official documentation for the radeon driver and it says HDMI audio is supported for Evergreeen cards since kernel 3.3, and RHEL 7.3 has kernel 3.10:

https://www.x.org/wiki/RadeonFeature/#index1h2

$ uname -a
Linux mediapc-rhel.localdomain 3.10.0-514.el7.x86_64 #1 SMP Wed Oct 19 11:24:13 EDT 2016 x86_64 x86_64 x86_64 GNU/Linux

Any ideas? That's about the extent of my knowledge on how to assess the issue so all suggestions, no matter how simple, very welcome.