I see a whole bunch of video connections and digital audio connections but...
...I don't see any analog audio (red/white) connections to or from the VCR. VCR audio is analog only and without these connections it won't work.
I don't know the specifics of that "tuner" you reference but generally if the VCR used the RF ("F" connector) as the input then normally the audio is piggybacked on that one cable.
For every composite video (yellow) connection into or out of the VCR, you'll need a red/white analog audio cable to carry the corrosponding audio.
So, pick up one or two of these and hook one up to the VCR as needed and see if that makes a difference.