Please note if you see the message "Passengers deboarded. Baggage loading in progress" very quickly after the deboarding started. This indicates the passenger process has terminated so, if it was too quick, no sounds were played.
If the process was too quick, it's because the airplane was empty. GSX reads the station load using the official Simconnect calls to get the total weight of all stations. Of course, if an airplane like the Q400, which a custom flight model, doesn't use the standard FSX variables to hold the station load, there's no way for GSX to know about the airplane load.
If this happens with any other airplane that uses standard variables, and I'm sure the QW757 is one of them ( like 99% of 3rd party airplanes ), the problem it's just that you haven't loaded the airplane, so it's considered to be empty, and this concludes the passenger boarding process very quickly (so no sounds will be played), even if the baggage animation is continuing, because they are two separate things.