It is option A that I meant.
The thinking is that if the speaker + is connected to Vcc, grounding that will short your supply, so don't do it. In normal operation the speaker - will be being pulled to ground by the circuit inside the door bell so you can use that to detect the speaker is producing some sound. You might have to add a pull down resistor from the speaker - and arduino input to ground, depending on the circuit inside the bell.
As long as the ground on the bells and the ground on the arduino are common you should be OK. So you should not be connecting the speaker + anywhere.