Hi,
I've had a similar desire so figured I'd share what I know as I haven't seen any better answer on the internet.
TLDR: the capture card firmware isn't considered an "audio inputs and outputs" device (it is a sound, video, and game controller) so to my knowledge you can't do what you're asking. This is true such that windows or any app can't really play JUST the audio but receives the feed as all or nothing (or something like that).
That said, from what I understand, the pass-through is passing the audio to the end device along with the video, so if you are using the HDMI out to a monitor that has an audio port or speakers, it's likely these would play the audio. The monitor I'm passing to doesn't, so I haven't been able to check myself. It's possible you could use an aux cable from your monitor back to your PC sound board to get the PC to recognize a source that's only playing the audio from the capture card, but I'm not sure if or how you could get this to work or what kind of latency this might have.
What I ultimately did for awhile was use the Avermedia RE central app with the multi screen feature so that it would only play the audio via audio preview, but this app kinda sucks and I wouldn't recommend using it. Half the time it doesn't even recognize my capture card despite everything being up to date and there being no issues. That said, if you wanted to use this app, you still could and would match the same setup below but substituting the RECentral App for OBS.
Alternatively, what I do now that uses the least amount of resources from my PC is as follows:
In OBS Studio I have a scene with just the capture card with preview disabled:
In advanced audio properties, I have the Audio Monitoring set to Monitor and Output:
*Note: If you just want to hear the source and nothing else, you could set to Monitor Only and skip the rest.
In System Settings I have output device set to a virtual cable from Voicemeeter. I believe you need to pay Voicemeeter for this, but I got it a long time ago and don't know if that's changed.
Finally, I have the Virtual Cable selected as a source in Voicemeeter:
At this point, any app you want to send the audio to should recognize the Virtual Cable as an audio source, and that Virtual Cable should be playing back the audio from your capture card. Even though this use case doesn't really make sense since the capture card source is already playing the audio, as an example you could now add the Virtual Cable as an "Audio Input Capture" source in OBS:
Or for a better example that I typically use, you can use the capture card as the device for a discord stream with the Virtual Cable as the audio device:
As for why the card doesn't work like we want it to, I got the below response from Avermedia support:
Thank you for choosing AVerMedia.
GC573 is a driver-based device, which means it functions differently from UVC/UAC devices like our other UVC device (e.g., Live Gamer 4K 2.1(GC575) . It is worth noting that enabling the Audio Capture function on the GC573 may lead to compatibility issues when capturing the surround sound channel. This is a result of the aforementioned hardware limitation.
We did considering for a firmware update for the GC573 to enable this option. Unfortunately, I must convey that we have concluded not to alter the architecture of the GC573. This decision is due to its FPGA-based structure, which would require an extensive period for modification and verification testing. Even though we've received similar requests like yours, I appreciate your understanding in this matter.
If you have any further inquiries or need assistance, please let me know, thanks for your understanding.
AVerMedia Product: GC573
Issue description: Device doesn't show up as an input or playback device. I would like to be able to play the audio only through windows (without needing to use RECentral) alongside the passthrough video so no system resources are used.
Though it would be nice, I see their point as it's a pretty niche use case (I had the capture card for ~2 years and never knew I wanted it until recently.