BOAC,
Sorry actually a type error!
The classic indeed can only have an intermittent horn in the air for a cabin altitude excedance.
The NG however, can have the intermittent horn on in the air for either a cabin alt exceedance OR a fault with the ground spoiler system.
The opening statement from the MM 31-53-00 page 2 reads
The takeoff warning function gives an aural warning sound if the airplane is in an unsafe condition during takoff or if the ground spoiler interlock valve remains open after takeoff
The conditions for in air warning; are ground spoiler interlock valve open and leading edge flaps and slats not extended.
Sorry for the thread creep.