Adding Audio to DVD

I have some old videos I have converted to DVD using my LG HDD DVD recorder. Unfortuanately, my father (now 75), in his wisdom had added an audio soundtrack to the VHS which I copied to DVD. If you can imagine a soundtrack of a hammond organ playing “like a puppet on a string” as a soundtrack you will understand the need to change it.

How can I add a soundtrack to this DVD to overwrite the original.

I have a burner and some software on my pc, so I can copy the DVD no problems. What software will take the DVD files and allow a soundtrack to be added. I do not want to convert the DVD files to AVI or MOV or MPEG. I want to just copy the DVD with a new soundtrack of some better music.

I have Nero 5, Clone DVD2, DVD Decrypter, Muvee Producer 4, DVD Slideshow. The last two are great but Muvee Producer 4 converts it to AVI and DVD Slideshow wont read a VOB.

Because the original VHS was pretty old the image is not good enough for AVI conversion.