If possible, Synchromesh will try to mix between different channel layouts. The only exception to this is if a clip has a 'discrete' channel layout - essentially this means 'don't know' - it's a custom channel layout that can't necessarily be mixed to one of the standard layouts. This will be mixed to the stereo outputs based on a very simple formula - Left/Ch 1 to Left, Right/Ch 2 to Right, and all other channels to both Left and Right. Use the channel gains (of the clip) to set gain and pan of each channel (up to 8).