Rip the whole DVD to your hard drive (as files, not an ISO or anything)
Find all the VOBs that have the show on (they should all be consecutive and in the same VTS), and drag them all together into DGIndex.
"File">"Save and demux video"
You will then have the original audio in whatever format it is, and the original m2v video saved as separate files.
Open the original audio in an audio editor that supports multi-track editing, import the new audio along side, and make the new audio syncronise with the old. It's important you do not change the old audio AT ALL. Make the new audio fit the camera audio, not the other way around.
Once syncronised, export the new audio (making sure the old audio is muted or deleted or something), and encode it to whatever format and bitrate the old camera audio was (Probably AC3 at some bitrate). Make sure the encode is DVD compatable - not all AC3 encoders produce DVD compliant files.
Re-author the new audio back with the old video, making new menus etc. You haven't actually touched the video at all, so if you did it right and made the new audio fit the old, it should syncronise exactly the same as the old camera audio did.