You wouldn't happen to be from England by any chance? I had to look up the word naf.
Assuming you want to completely replace the commentary with music instead of mixing the two, it shouldn't be too hard to do this. You'll need to demux the audio and video streams from the dvd-video. (Demux means to separate the audio and video). The easiest way to do this is with PGCDemux, which is free and can be found here: http://www.videohelp.com/tools/PgcDemux
To demux, simply start PGCDemux and look in the Video_TS folder of your dvd movie. It will want one of the IFO files there. Enter the VTS_01_0.IFO as the input into PGCDemux. Make sure to put a check mark in the box next to Demux video stream. Then set a place on the hard drive where you want the streams as Output, then click Process to start demuxing.
So now you have the video separate. The video will have an .m2v extension on the file.
You can import this .m2v file into an editing program like AviDemux. AviDemux will recognize that it is looking at mpeg video and will ask to index it. Let it do so. You can now do simple edits with AviDemux...cutting out sections will be about the extent of what you can do with the program. Set marker A at the start of the cut, set marker B at the end. Go to Edit and click Cut to remove the section you don't want. AviDemux can be found here, and is free to use: http://fixounet.free.fr/avidemux/
Now you are ready to import your audio stream. You'll need audio that is similar in length to the video, else one part will end while the other continues. Click on Audio in AviDemux, then Main Track to import the audio. The external audio stream can be in ac3, .wav or .mp3 format.
Now on to the output settings for your video. This is where it gets a bit tricky.