I stand corrected - once I added an actual moving window part to the cab, the audio worked as described. About the only thing I'd change is ask that the locomotive's external SMS be blended with the internal SMS when the windows are open.
One minor correction, naming the mouse click parts LEFTWINDOW or RIGHTWINDOW will not work. The correct names are ORTS_LEFTWINDOW:#:# and ORTS_RIGHTWINDOW:#:#. In my case, the names are ORTS_LEFTWINDOW:0:0 and ORTS_RIGHTWINDOW:0:0.