In OBS Studio when you do not see your source in the Audio Mixer, that can be because the source is disabled (the eye icon in Sources) or because the source is in Preview, not active in Program (when in Studio Mode). If the source has audio capacity but no sound (ie. an mp4 without audio track) you would see the source at the Audio Mixer, just no bar movement.
You might have a look at MS Windows' settings, Devices and check whether the audio device of the video grabber is disabled.
If there is no sound from the video grabber going to the computer at all, you'd have to look at it's documentation. I had a quick look at
digitnow.com but I couldn't find any manuals for download (obviously I might have been at the wrong product pages). Hopefully their support can be of any help.
