OBS considers Desktop Audio in Windows, to be the default audio playback device. The default audio playback device is chosen and enabled in Windows Sound Control Panel. Applications by default send audio output to the default audio playback device. They may also provide audio settings to let you choose audio devices that are in the Ready state.
What I would recommend is this:
- Set the volume knob on your speaker/headphone to say 50%
- Set the volume level in Windows Sound Control Panel for your audio devices to be 99 or 100. Don't change it to avoid affecting all the applications that reference them (next step)
- Customize each application's volume level within its audio settings
This way you have a fixed audio volume baseline that won't change when you record and stream over time.