I forgot to mention: my audio system uses PCM input over HDMI arc. But outside of the Steam Link program it's working as it should...

HDMI is plugged into the TV and then an arc HDMI cable from the TV to the receiver.
The audio settings are already set to stereo... are there any logs I can review in order to get an idea what's going wrong?

By the way: I've also got a Steam Link hardware device and the audio there is working fine - so it shouldn't be my Windows PC...

Just a follow up:
No, I haven't tested the audio jack of the Pi, but I changed many many things in the Linux system and now it's working as it should. Main thing I've changed so far was to install and configure pulse audio, but I doubt that this fixed anything...
Anyway, it's working now! Thank you for your help :)

Checking in from 2020 - the issue for me was that I was using a newer OS than Stretch. Using a fresh install of the 2019-04-09 build from https://downloads.raspberrypi.org/raspbian/images/raspbian-2019-04-09/, skipping any OS updates during initial boot, and then installing steamlink per usual worked like a charm. Cheers!

Is there a solution to this that does not include erasing my entire setup?

I accidentally exploded my Raspberry Pi 3 and got a Raspberry Pi 4 to replace it. It took days for me to get the 4K60 option working and installing Kodi etc.
I'd rather not reflash the entire card again.

It used to work.

Originally posted by slashreaper:

Checking in from 2020 - the issue for me was that I was using a newer OS than Stretch. Using a fresh install of the 2019-04-09 build from https://downloads.raspberrypi.org/raspbian/images/raspbian-2019-04-09/, skipping any OS updates during initial boot, and then installing steamlink per usual worked like a charm. Cheers!

This was it! You are the man! Thank you!!