If the VCR has a tracking control try adjusting that. If you have a VCR cleaning tape you can try that too.
Leave the VCR connected for video so the TV will allow you to select the input (some won't unless there is video present) and connect another source of analog audio (phone for example). That will tell you if the VCR or the tapes are bad.
It is connected in the regular way - RCA to the adapter that came with the TV and then into the av port on the back of the TV. Tried it again yesterday and suddenly had audio - but went back last night - same tape - and again no audio so I think it must be the VCR itself. Other suggestions welcome.