1) During preliminary flight deck preparation the system is checked for functionality, as you KNOW the sound MUST sound as you are on stand, parking brake set, flaps up.
2) After having configured the aircraft for departure, prior to taxi or in freezing conditions after configuration of flaps prior to lineup, the config check is done to ensure correct configuration for departure.
3) Any subsequent takeoff config during departure on the runway means step 2 has been missed and thus aircraft is not properly configured and requires an RTO to establish the reason
In the outfit described a return to stand is mandatory after a config on the runway, previous steps prevent this from happening IF following SOPs.