(This will only work if your AC3 decoder applies dialogue normalization. That should be the case for all DVD players and hardware decoders. Software DVD players I'm not so sure of, but it does seem to work with PowerDVD.)
1. Make 2 new folders on your hard drive (named "original" and "fixed"). Copy all the files off your original disc into the first folder.
2. Download the program VOBDNorm from here: ftp://ftp.coises.com/vobdnorm-1.0.zip, and extract the exe file into your command path (e.g. c:\windows\system32).
3. Open a command window, and navigate to where your original and fixed folders are located.
4. Type the following command:
VOBDorm original\VTS_01_1.VOB fixed\VTS_01_1.VOB 31
(Note I've used 31 here for maximum volume. Strictly it should be around 27, if I've measured correctly.)
5. Repeat step 4 for VTS_01_2.VOB up to VTS_01_5.VOB.
6. Copy the IFO and BUP files from original into fixed.
7. Now burn the contents of the fixed folder as a DVD-video with Nero, etc... (you might want to use a re-writable disc first just to make sure this method works)
Let me know how you get on!