For those of you interested in putting the riff-track on the rear surround-sound speakers, here's the procedure I used with free software. I really like this system because you can control the volume of the riff separately using your receiver surround sound settings.
- do steps 1 and 2 in the official guide above (rip DVD files and fix audio track, you should read full official guide to get details on the full process)
- download Audacity 1.3.6 (I got it from audacity.sourceforge.net)
- go to Edit|Preferences, Import/Export tab, download FFmpeg library
- on same tab, in box "When exporting tracks to an audio file", choose "Use custom mix"
- load in fixed AC3 file (from step 2 in official guide), it should load in 6 channels which I believe are: left, center, right, rear-left, rear-right, subwoofer
- import rifftrax mp3 file, should load as track #7
- in Edit|Preferences, Quality tab, set sampling defaults to 48,000 Hz and 16-bit (default for DVD AC3)
- align rifftrax (channel 7) with main audio (channel #2) using time shift, silence sections as desired, copy/paste silent portion to the end (join any splits)
- select entire movie track, all 7 channels, from beginning of DVD audio (the rifftrax will start earlier if you used the time shift method above)
- select File|export selection and choose WAV format
- click OK on the Edit Metadata screen if it appears (there's an option to make it not appear)
- on the Advanced Mixing Options screen, choose 6 channel output and map the tracks (by clicking diagram) as follows:
1st track (left) to channel 1
2nd track (center) to channel 3
3rd track (right) to channel 2
4th track (rear-left) to none
5th track (rear-right) to none
6th track (subwoofer) to channel 4
7th track (riff) to channel 5 and channel 6
* this ordering is necessary because the next step changes the order, and this makes it go back to the original DVD order
- use AftenGui (with aften.exe, I got both from videohelp.com) to encode the 6-channel WAV into 5.1 AC3 (I used all the defaults)
- use the newly created AC3 to make DVD folder with IFOedit or rejig (step 6 in official guide)
- option A: use DVD shrink to make iso file for burning (final step in the official guide)
- option B: use ConvertXToDVD (commercial software) to make DVD file with a menu screen (and shrink to fit on single-sided DVD); then use imgtool.exe to create iso file for burning
Good luck!