As for you, I think it makes a lot more sense to just speed up the music tracks and then sync that to the unmodified DVD. No sense slowing down the DVD, then speeding up both the DVD and isolated track at the end of the process when you only need to make one change. You'll experience less of a quality drop speeding up the audio than slowing it down. Just make sure that when you speed it up you preserve the pitch.
The only reason I'd suggest doing it the way you posted was if you were going to release an NTSC copy, but if that were the case I'd recommend you just get an NTSC copy of the film, do all your work, then speed the isolated track for resynchronisation with your PAL copy.