Comment 2 for bug 884336

It also works if I switch the Pulseaudio output configuration to "Digital Stereo (HDMI) output" and back while the sound is playing. As soon as the first change is done, I hear sound (in stereo, of course) and after the second change I have nice 5.1 sound.

So Pulseaudio is clearly able to do everything necessary to have sound, it just needs to do it automatically when starting playback.