If you are using a cable like
this to connect the converter to your VCR, consider using a cable set like
this instead.
I am presuming that the model number of your VCR is actually DV-T3DN and that it is not capable of stereo recording or play back. If I am wrong, and you have stereo capability, a cable set such as
this would be able to support stereo.
My thinking is that there is a problem with the video and audio being combined and sent over a common RF channel. This change would have you record from the line input rather than channel 3 or 4.