Problem solved but I don't know what I did - can anyone explain?

[Chris Angelico]

Hi! Sorry if this is the wrong place to ask this; I didn't find a
matching -users mailing list.

What I'm about to lay out is my struggles with audio over the past
week. But now suddenly the problem is solved... and I don't know how.
Which means, if I run into the problem again, I won't know how to fix
it! Please, can anyone tell me what happened here?

I have a number of audio devices in this computer, including a
headset, a set of speakers for alerts, and a set of high quality
speakers connected via S/PDIF (TOSLINK) to a breakout off the
motherboard. I recently did a fresh install of Debian onto a new SSD;
the previous install drive is still available so if there's any config
files to compare against, I can fetch them.

The S/PDIF works fine in ALSA. To get audio routed to that output in
ALSA, I send it to "USB Audio, USB Audio #2 Hardware device with all
software conversions" in VLC, or with "aplay -D
plughw:CARD=Audio,DEV=2". No problems at all.

But in Pipewire or Pulse, that device didn't show up. There are a
number of profiles available to me, as shown in pavucontrol or any
other place that lists device profiles, but none specify this device,
and playing audio through all of them leaves the speakers silent.

On the previous installation, I would use the sink
"alsa_output.usb-Generic_USB_Audio-00.HiFi_7_1__hw_Audio_2__sink" to
send audio to the S/PDIF. Nothing I've done has made any such device
name appear.

As of now, though, I have a new profile available in pavucontrol,
which hadn't previously been there: "Pro Audio". This makes available
four sinks called "alsa_output.usb-Generic_USB_Audio-00.pro-output-0"
to "pro-output-3", and output-2 results in the S/PDIF lighting up!
Yay! But what made it appear?

My best guess is that installing OBS Studio build dependencies may
have done something. I'd previously been running OBS as installed from
Debian's repos, and now am about to start building from source,
although I hadn't actually done the build yet - just installed the
build deps and cloned the repo.

Any clues as to why this is now suddenly working?

ChrisA