I am getting the exact same bug. Local play works fine, but remote play doesn't even create an audio stream in pavucontrol
Edit: exiting the steam families beta and returning to stable branch solved this issue for me
Yep, i have noticed this as well. I tested on my Laptop running Ubuntu, Steam Deck, and Macbook. All 3 on the Steam Beta Client. The Macbook streams audio just fine, but both the Linux clients fail in the same way.
PulseAudio: Sink alsa_output.pci-0000_04_00.0.hdmi-stereo is default
PulseAudio: Adding sink[937] alsa_output.pci-0000_04_00.0.hdmi-stereo with priority 7, monitor is alsa_output.pci-0000_04_00.0.hdmi-stereo.monitor
PulseAudio: Recording on device alsa_output.pci-0000_04_00.0.hdmi-stereo.monitor
Audio mix: start=430365012633, returned=0
Audio source [System Pulse]: init=0, adjustment=0, through=0, last_start=0, mixed=0, drop_before=0, drop_after=0
PulseAudio: Connected to device alsa_output.pci-0000_04_00.0.hdmi-stereo.monitor (937, not suspended).
PulseAudio: Using sample spec 's16le 2ch 48000Hz', channel map 'front-left,front-right'.
CLIENT: Got control packet k_EStreamControlSetQoS
CLIENT: Got control packet k_EStreamControlSetTargetBitrate
BuildCompleteAppOverviewChange: 756 apps
Switching video stream from NONE to GameOverlay_MovieStream_467056
CLIENT: Starting audio stream: frequency=48000, channels=2
CLIENT: Couldn't open audio device: format=2, channels=2, frequency=48000. Audio is disabled: Source channel map is invalid
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
While streaming from a windows host to an arch linux client no audio can be heard, everything else works fine.
Audio is played on the correct speaker on the host (as it's the only one, I've unplugged everything else):

Nothing can be seen on pavucontrol:

So far I've tested with tf2, astroneer and just cause 3, all three do not work
Steps for reproducing this issue: