About audio monitoring in advanced audio properties:
OBS can output audio to two different destinations: one is called "output". Whenever you read "output" in OBS, it means audio is put into the video file OBS creates for recording, and to the video stream for streaming. The other one is "monitor". Whenever you read "monitor" in OBS, it means the corresponding audio is put on the monitoring device you set in Settings > Audio > Advanced > Monitoring device. If you read "default" in there, it's the default Windows desktop audio device, usually speakers, so you will hear that.
In advanced audio properties, you decide this audio destination: either monitor (and not output), or output (and not monitor), or both monitor and output.
If some audio within OBS is silent for you, it may be it's generated within OBS and not monitored. But it might be output to the recording, so you will hear it if you watch the file in your media player.
If you activate monitoring for some source you find silent, this audio is put into the Windows audio sound system, so you will hear it with your speakers or headset. If you capture that speakers at the same time with OBS, you capture that sound twice: once directly and a second time through capturing the monitoring device. Usually, this double capturing will result in echo and feedback loops. OBS tries to avoid this feedback loop by deactivate monitoring for such sources. In this case, activating monitor for some source seems to do nothing.
Capturing audio from a capture device can be configured in multiple ways in the audio settings of the capture device source. Scroll to the bottom and see all the options by playing with them. The default setting is "capture audio only" will just put the audio within OBS and will not monitor it. That's the best and most direct way. It will appear silent for you without monitoring, but the audio bar in the mixer will move, so there's proof there's audio. If you record this, you will have the audio in the resulting file. If you activate monitoring for the capture device, you will also hear it within Windows, but only if you don't capture Windows desktop audio at the same time. So I recommend you set all global audio devices in Settings > Audio > Global audio devices to disabled. If you intend to speak a commentary, you can leave the mic device enabled, but both desktop audio devices to disabled. You will also avoid Windows sounds being carried over into your video recording this way.
You seem to have done this, because Windows desktop audio isn't appearing in the OBS audio mixer for you. It's not clear for what you added an audio input capture source. This source is made for mic devices. If you don't use a mic, you don't need that source. You already configured a mic in your global audio devices, since Mix/Audio is appearing in your mixer - I doubt you have 2 mics, so at least 1, probably both mic sources are not needed and can probably be deactivated or removed.
If you just want to record audio from the capture device and nothing else, just the capture device audio source should appear in the mixer. Remove everything else, and you will avoid any influence from unwanted sources and the confusion that might arise from that.
