>export the files into MPEG2-DVD... I multiplex the sound into the MPEG-2 file
Those two statements are mutually exclusive (or, they should be !!!)
The PPro MPEG2-DVD preset is designed to create TWO output files... one video and one audio... which you then import into Encore, at the same time, for DVD authoring
When you multiplex into a single file, Encore has to do the added work of separating the audio/video for authoring
So... which is it... are you creating two files with the MPEG2-DVD preset... or not?