funny that you post this, as i was considering starting a thread about the exact same thing. i do a lot of SBD/AUD matrixes using stretch in Vegas and on a few i've begun noticing the same thing, i was going to describe exactly as you did -- phasing. sort of like the sound is blowing from one channel across to the other at times. i thought maybe my ears were just playing tricks on me, good to know it's not just me. the problem is not that they aren't fully in sync -- they are definitely synced as well as they can be. i'm no audio expert but i was wondering if it maybe has to do with how the resampling of the 'stretch' function is working. i can provide samples also if it would help. funny thing about it is i don't notice it in every matrix, only here and there. and it doesn't affect an entire show, just pops in for a few seconds then goes away. very puzzling. and it is noticed better on headphones than speakers, as you said.