Hi Bruce,
After you export to AVI from Premiere, use VirtualDub to extract your audio.
Make sure you use "Full processing mode" on vDub, on the audio tab.
Now encode your AVI->MPEG-2 with an .avs script, so it only gets the video from AviSynth, and the audio from the extracted WAV file.
Your .avs should look something like this:
Code:
AviSource("C:\YourAVIFile.avi", false)
That will feed your encoder with the video portion only, so you can choose the external WAV audio source which should be in sync with the video, as extracted by vDub.
Give that a try
-kwag