You will need to demux the audio from the DVD using an app such as DVD Decrypter, Smart Ripper, DVD2AVI, etc. You they need to convert the DVD audio to a wav file. Since the audio most likely will be .mp2 or AC3, I would use Headac3h to do the conversion, although Besweet could be used too. Once you have your audio in .wav file format you will need to edit the audio in a wav editor. Cool Edit Pro (now Adobe Audition), Soundforge, Goldwave, etc should do the trick. You can normalize to get your “equal audio”, but you’ll probably have to manually go through the file and tweak certain parts. If the camera noise has a certain sound characteristic, it can be filtered out quite easily. Once your done filtering your audio, convert it back to whatever the original DVD audio format was (MP2 or AC3).
You’ll most likely have to reauthor your DVD as I’m not aware of a simple way to mux the video with the new audio while adding the padding streams. If you want to maintain your current menus and DVD structure, just encode your audio at the same bitrate as the original file, author up a DVD with the new audio without menus. Now go to the output of the new DVD and delete all the files except for the big VOB files. Drop the same files from the original DVD (the same ones you just deleted from the new DVD) into the folder with the new DVD files…and you should now have a DVD that has the original menu, chapter points, etc with the improved audio.