Sound over HDMI used to work always, after selecting HDMI output. Today, however, I changed the mode (from stereo to 5.1 surround) and since then I have the following:

  • I can't change the 'mode' anymore
  • The 'Test' buttons in the sound settings do work
  • As soon as I start to play sth via Firefox, there is no sound
  • When I then use the Test buttons after half a second, the sound works again (I can hear "... right" the first time, after I can hear "front right")

Anybody a clue what's going on? Or what I can do to find the faulty? I'm on 16.04 and https://help.ubuntu.com/community/SoundTroubleshootingProcedure Step 1 didn't do anything.

EDIT 1: Just discovered that the sound over HDMI does work properly if displays are mirrored, whereas it doesn't if the laptop screen is uncoupled from the TV (like, different resolutions, different windows), of if the laptop screen is off.

EDIT 2: Things are getting even weirder: I now have the screens not mirrored, in sound settings selected HDMI. When I play video a in Firefox does create sound, video b in Firefox doesn't. Also: Playing an mp3 file with Videos (default programme in Ubuntu) doesn't play on its own, but when the video a is running in Firefox (and thus I have sound on my TV) and I play the mp3 file again I can hear it mix in as expected through the sound of the video. Does this indicate some automatic switch/mute mechanism, which I might be able to disable?

  • How did you change the mode? – user689314 May 28 '17 at 17:54

Have you tried installing alsamixer and then going to Select Sound Card by pressing F6 and check if your Sound Card is there and make it default ?

  • Hadn't tried, there only is - default and 0 HDA Intel PCH -> can't seem to select one specifically. In any case, just discovered the weird thing that sound does work when screens are mirrored (without changing the card). – Koen May 28 '17 at 8:27

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