An IDC (Initiating Device Circuit) is wired the same as a NAC (Notification Appliance Circuit). It's the wires and not the devices attached to the wires that are talked about with Class A or with Class B.
The only wiring difference between Class A and Class B is that with Class B, the end-of-line resistor can be anywhere in the building, while with Class A, the end-of-line resistor is back at the panel, usually on the main circuit board. (It's there, usually you aren't told about it.)
With Class B, any break in the wire will show up as a trouble on the panel, and any device beyond the break in the wire is useless. This break, of course, will be fixed in a timely manner.
In a fire, a timely manner isn't good enough. To keep the devices working that are beyond the break, the panel switches to feeding power or audio on both the Class B feed and the Class A return part of the loop. When there's a fire, this back-feeding on the Class A to get power or audio to the devices might just save lives.
Besides the length allowed, and the diameter of the wire, there is no difference between an input circuit (IDC) or an output circuit (NAC).
Any input device (pull station, smoke detector, waterflow switch, etc.) is attached to the Initiating Device Circuit (IDC). Any output device (horn, strobe, speaker, etc.) is attached to the Notification Appliance Circuit (NAC).
The only difference between a horn circuit and a speaker circuit is that the horn circuit is fed 24 volt power when there's an alarm, and the speaker is fed amplified audio (like any distributed sound system) when there's an alarm.
There are addressable modules that can be used to send audio over Class A circuits, but I haven't seen any complete audio panels yet that are specifically designed to do Class A. They probably exist, but I have never had to research them. You'll have to research to find them yourself.
One other place you can get information is the technical support people at the fire alarm system manufacturer for the rest of the system.