Ok, for those of you who just want it up and working, I’m including a quickstart section before we dive into the details:
1) If you have an ATI/AMD or NVidia card, you need proprietary drivers.
2) You need to activate your secondary screen. For Intel, this is done in the regular “Screens” dialog, and on NVidia this is done in the nvidia-settings dialog. (I haven’t tested fglrx.)
3) You need to select the HDMI/DisplayPort output in the sound settings dialog, which is quickest reachable from the sound indicator.
Can’t we switch audio output automatically?
Choosing whether to automatically switch to HDMI/DisplayPort – essentially, switching sound to use the HDMI/DisplayPort whenever that screen is activated – is not trivial. It is not obvious to me whether the user wants to do that, or not. And in fact, in Ubuntu 11.10, we did switch, but only for some cards. And we did not switch back when the screen was deactivated. After a discussion where different opinions were voiced, I reached the conclusion that given the current pieces of infrastructure in place, the best option would be to disable automatic HDMI/DisplayPort switching for Ubuntu 12.04.
The problem of four devices
As mentioned in an earlier post, much HDMI/DisplayPort hardware have phantom outputs, and there is no way we know what outputs are real until something is plugged in. With the new sound settings UI in Ubuntu 12.04, we finally have a good user experience in this scenario: Only the outputs that are actually plugged in and possible to activate will be shown.
Sound settings in Ubuntu 12.04
Most of the code to activate HDMI/DisplayPort audio is in the video driver, rather than the audio driver. Therefore, if this is not working, it is more likely that the problem is within the video driver.
It is also notable that the open source driver for ATI/AMD (called radeon), has experimental support for HDMI/DisplayPort audio, at least for some cards. It is disabled by default, but you can activate it by adding radeon.audio=1 as a kernel boot parameter.
PulseAudio 2.0 is soon to be released (hopefully). PulseAudio 2.0 and Ubuntu 12.04 have the same feature set when it comes to HDMI/DisplayPort audio support.
The new sound settings UI in Ubuntu 12.04 has not yet been upstreamed.