Not sure what the deal is, but if I were trouble shooting this problem, I would start by minimizing the 18 audio tracks. If the audio mixdown is giving you an echo, then I would say it has nothing to do with your export settings..unless you are saying the mixdown is fine on your timeline, and echos only on export?
Either way, turn off all but the 2 main audio tracks and try an export. Bring that quicktime back into avid and sync up with your timeline…is it in sync?
What happens if you export your audio tracks in groups of 2 as a wave file, and re-import them…are they in sync with your original timeline
How long is your sequence?
Have you tried exporting only the second half? Does it stay in sync roughly the same amount of time before drifting. Does it start drifting at the exact same spot in the sequence.
Check your export settings, or try better yet make new ones, using a different codec, that you build from scratch.
Are you sure you are properly monitoring all 18 channels when you hit play…confirming there is no echo on your timeline.
(really sounds to me like you have some audio doubled up and slipped somewhere, and you simply aren’t hearing it on playback, but you do hear it on export)
Glenn