You can also rip the audio track from the vobs in the videoTS folder (demultiplex) and re author into a new DVD with a new sound track.
Yes, Wagsy's right on here about ripping your own DVD. If you really really need the movie, and the only source you have for it right now is the DVD you made, then try this:
A good shareware program for ripping/demuxing DVDs is MPEG StreamClip. Download and install it, then double-click on the icon for your DVD (quit DVD Player and just open up the DVD disc icon, itself).
Open up the Video_TS folder and find the files that end in VOB. Those are the various video assets in your DVD but they're "muxed" (multiplexed) into a DVD structure, so you need to de-mux (de-multiplex) them, or separate the video and the audio back out, then convert them back into DV files that you can then re-import into iMovie to tinker with.
StreamClip gives you a number of different options for converting the VOB files. It's pretty intuitive. You probably want to reconvert to DV and work with your video and audio again in iMovie? Then try that option in StreamClip.
Sometimes it isn't obvious which of the VOB files is your movie (remember, the motion menu will be a VOB file as well). So just try each of the VOB files in StreamClip until you find the one that's your movie.
I use iDVD sometimes for fast jobs, and I like some of their themes. It definitely has its limits, and DVD Studio Pro is really the program you want to get conversant with. But iDVD will usually work OK for really fast, simple jobs where you don't mind the limitations of the themes, or don't want to take the time to create your own menus, etc. Just make sure you set your preferences in iDVD so that you're getting the best quality encoding, though.