I tried the following steps to get my HDMI sound output working under 14.04:
- GRUB_CMDLINE_LINUX="radeon.audio=1" in /etc/default/grub
- sudo update-grub
- Go to ALSA daily build snapshots
- Select "oem-audio-hda-daily-dkms - 0.201404230703~ubuntu14.04.1"
- In "Package Files" download the .deb-file
- Install the .deb-file on your system
- Restart the system
This doesn't work for me. FYI my Radeon is a 5750 series card.