To know the real answer to your question about speakers and horns, some background information is needed.
Remember that a fire alarm system is a "Detect Fire and Warn People of the Danger" system; it's a life safety system. With that in mind, a fire alarm system has to detect all possible fires in a building, and then warn all occupants of the danger, and do this with no down-time. Basically, if there's a fire, the fire alarm system sounds the alarm.
Classification, however, isn't really the exact method of installing wires, classification is knowing how the system deals with failures. When something goes wrong with the system, does the fire alarm system warn the building owner that there's a problem? Is there a method to get around the problem so the system will detect-and-warn even if there's a problem? These are the real reasons for classification, so that the fire alarm system designer, the system installer, the fire marshal, the building owner, the insurance company, the repair technician, etc. can know exactly how well the system is going to continue to detect fire and warn people of the danger.
If the fire alarm system, as a whole, can't detect all the fires it was designed to detect, or if the fire alarm system can't warn everyone of the danger, the panel will turn on its trouble light and the trouble buzzer. The trouble light and the trouble buzzer are there to say "I need to be fixed now".
In all this, all detection devices, and all horns, strobes, and speakers have to be connected to the panel to detect fire and sound the alarm. If they aren't connected, they cannot work. Wires are the usual means of connecting the devices to the panel. If a wire breaks, or comes loose from a connection, one, some, or all of the devices on a circuit won't work. Yes, sometimes wires break or come loose, and when that happens, whole areas of a building aren't protected by the fire alarm system.
Wiring, though, is hidden in the walls and ceiling. Even the wiring connections are hidden behind the horns, strobes, and speakers. When a wire breaks, when a wire comes loose from a connection, no one is going to see the damage. Without some means of telling the building owner that there's a problem with the wiring system, the building owner will assume the fire alarm system is working . . . even when the fire alarm system is broken.
Rather than waiting for a fire to occur to find out that there's a problem with the wiring, the wires are supervised by the panel.
A fire alarm's internal supervision, in this case, means "Continually watch for a problem, and provide a warning when a problem occurs". When the wires are installed so that they can be checked for continuity, if a wire breaks or comes loose from a connection, the panel can turn on its trouble light right away. The building owner can then call for service.
The idea behind Class B classification of the NAC wiring systems is to make sure the system will be fixed in a timely basis - before a fire occurs.
If a problem with the wiring can't be fixed soon enough, or if the wiring comes loose during a fire, the idea behind Class A classification of the NAC wiring systems is to make sure the most or all of the horns, speakers, or strobes still work.
Speakers are the Same as Horns
For the people who occupy a building, there is no difference between the sound from a fire alarm horn and the sound from a fire alarm speaker: they both sound the alarm. The power or signals that are carried by the wiring system may be different, but the wires themselves are the same. They share the same kind of copper and the same kind of insulation. If the wires for a fire horn can go bad, the wires for a fire speaker can go bad.
The bottom line is that the same classification rules that apply to fire alarm speaker wiring are the same exact rules that apply to fire alarm horn wiring. They are both Notification Appliance Circuits (NACs).