Librem 15v3: sound through HDMI cable

[白い熊]

Hi Giux:

I'm running GuixSD on a Librem 15v3 — this seems to me an ideal setup: the only 
completely free notebook with the best GNU free system! :@)

I have a simple use case - I’d like to hook up my TV to the Librem 15v3 with an 
HDMI cable and watch movies on the TV from the notebook.

It outputs video fine - the picture displays on the TV - but the sound comes 
out from the notebook’s speaker.

If I boot up the Librem under it's default PureOS — it works fine. I’ve 
confirmed that when I plug in the HDMI cable I see “HDMI / DisplayPort (plugged 
in)” as a “Port” option on the “Output Devices” tab in pavucontrol. When it’s 
selected, sound does go through! :@)

Now, I’d predominantly like to use GuixSD as my OS on the Librem. Here however, 
when I plug in the HDMI cable I don’t see the “HDMI / DisplayPort (plugged in)” 
as a “Port” option on the “Output Devices” tab in pavucontrol. I only see the 
default “Speakers” and “Headphones (unplugged)” options — the same as under 
PureOS when no HDMI cable is plugged in. I do however get the video through, 
but not the audio. It stays on the PC speaker.

Would anyone have an idea on what’s causing, what I could set up in GuixSD to 
get the HDMI cable as an output option — so I could get the sound to the TV?

--
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白い熊