HDMI audio works but does not show up in GUIs

Like others I am having problems with outputting sound over HDMI (e.g. #1158909), however in my case the hardware clearly works, it is just the the HDMI option doesn't appear in the GUI etc.
   aplay -D hdmi ...
 works, and I can get all the programs to output to hdmi correctly by running
   echo "set-default-sink alsa_output.hdmi" | pacmd
However, I would expect a way to set the output to HDMI via a menu, but none of pavucontrol, xfce4-mixer or alsa-mixer allow this.

I am using Lubuntu Raring 13.04 AMD64 with kernel 3.8.0-15.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: pavucontrol 1.0-1
ProcVersionSignature: Ubuntu 3.8.0-15.25-generic 3.8.4
Uname: Linux 3.8.0-15-generic x86_64
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Mon May 20 22:41:45 2013
InstallationDate: Installed on 2013-04-03 (47 days ago)
InstallationMedia: Lubuntu 13.04 "Raring Ringtail" - Alpha amd64 (20130401)
MarkForUpload: True
SourcePackage: pavucontrol
UpgradeStatus: No upgrade log present (probably fresh install)