If I remember correctly (if your're musically -inclined (and I'm not)) the seat belt chime is close to a middle C in the musical scale.

Remember the chime code (seems to be pretty universal). Usually, this is done only by the flight crew cycling the FASTEN SEAT BELTS/NO SMOKING light switch in the cockpit:

2 chimes - passed through 10,000 feet; flight attendants may begin meal/beverage service.

4 chimes -- cleared for takeoff/landing, flight attendants must be seated in their jumpseats for takeoff/landing.

Other chime codes are prearranged between the flight crew and flight attendants, and are usually reserved for emergency situations.