Every doorway will have a panel (with some units two - one on each side of the door, but not usually so with 321s). The sound produced by the sounder when the door is unlocked / about to close is controlled centrally, but the actual noise it produces is determined by the hardware inside the panel itself. As 321s use basic analogue components to produce their sound, variations occur in pitch, speed, volume etc. - this is what is giving the examples you listed their distinct sound. My point is, this effect means that if you, for example, travelled to the next carriage along, or even just one of the other three doorways in the same carriage, you'd potentially hear a different sound. Thus, it'd be impossible to tell which unit is which by variations in that sound, unless you always knew to stand by one specific doorway.