Well if it's in mp2 format than you're not following the advice of your own quote are you?
Actually, the reason is that mp2 is not part of the dvd disc standard for region 1 (north america) DVD's, but almost always works anyway. In any case, I'd try 1 of the following options:
1) convert the mp2 to ac3 with dBPowerAmp (freeware). You may need to download additional codec's from their site to do this
2) convert to wav with the program of your choice (such as Audacity or Winamp), and then convert to ac3 with besweet or belight (this is the more complicated option)
After audio conversion you'll have to reauthor your dvd
poke around vcdhelp.com if you need more help with this