I have the same issue. I listen to the games audio through my speakers, and the only way to lower the volume is through the Windows Volume Mixer and lowering the audio for OBS, however this also lowers the volume for everything else OBS picks up through the computer as well as any browser sources that have audio such as alerts.
In OBS Classic, when you went to the capture device properties, you had the option to lower the audio from there, but that option only exists in Studio if you are only streaming the audio and not outputting it to the desktop. Is this something thats going to be added at all or is there already a way to do it that Im not aware of?