They need to have matching timecode. I typically do this by digitizing the digibeta source uncompressed and then syncing it with the raw AIF mono tracks from the mix in FCP. From there you export the final MOV file matched in length by synced up mono AIFS. The MOV goes into Compressor for MPEG-2 encoding, the AIFs go to APack or Compressor 2 to become a synced AC3. If you start with just the AC3 mixdown there’s no way to sync it other than guessing.
Noah