So audio is my weakest area... you are warned... but in case you don't get another better , response:
Uh, have you checked that the camcorder is outputting audio over the RCA connection? Do you have audio level turned up on camcorder?
to test, what I'd do is hook up camcorder to a TV and see if video and audio working as expected.
If yes, then that capture device is possible culprit.
Next step would be to see if Windows getting audio .. With Windows 10, audio has lots more settings, and add-in audio software (Realtek and others) can cause unexpected behavior. lots of threads where is mis-configured Windows settings, including Privacy related causing audio input issues
Once you know audio getting to Windows, then work within OBS to 'hear' the audio