Steve G: David is right, because the (now) normal HiFi audio track occupies the same space on the tape as the video data, so you can't alter the audio without affecting the video signal. What's left is the linear audio track on the edge of the tape which is available for alteration. Only a few very expensive VCRs had that feature in the history of the format, and none exist today on the general market.
Your only practical alternative is to copy the video to another machine concurrant with different audio. This would, unfortunately, mean a degradation of the video quality on the resulting copy.
Hey there Steve... There are a few ways you can go about dubbing new audio onto an existing vhs... I don't know what you're trying to accomplish with the dubb, but one of the simplest ways would be to look into getting a used video camera that has an over-dubb option on it. This would be far easier than trying to locate a VCR that will do the same. (I use a Sharp VL-l63U camera for my dubbing needs) I hope that might be of some help to you. If you have a more specific question, you can e-mail me with it if you'd like... Sincerely, Michael