Hi John,
I have noticed that the speaker/siren makers call them speaker or siren not based on the output sound, but in the way that you drive it. If it only has the speaker, then it is a speaker. If you take the same speaker and add an internal electronic module and amplifier that makes the siren sound to drive the speaker, then it is a siren.
In the case if OUT1, you can only connect to it a speaker because it is used to produce voice sounds in addition to the siren sound (the sirent sound is produced by the control panel). In the case if OUT2 you can have the Elk control panel produce the sound and connect a speaker to it. Or you can have the Elk control panel just output a 12 volts signal and you have to connect a siren that will produce the sound.
The speaker included with the M1G package yields a decent siren sound, but it is installed inside my house and cant hardly be perceived from outside the house. At least, not from far. For this reason I'm planning on installing another speaker or siren to OUT2 and install it outside the house.
If your have OUT2 with a siren/speaker outside of your house I was wondering what would be the value of having OUT1 driving a siren sound - specially if it cannot be heard outside. Later I found an idea in a securityweb page (I dont really remember where) that said that you want to make the stay in your home as annoying as possible for the burglar. A siren driving 120dB in the room where you keep your valuables is a good dissuasive to convince them to hurry up and abandon your house.
Currently I have only one speaker ( the 73) installed above the stairway. I already have two more speakers that plan to install in parallel to OUT1 (up to 4 can be connected) to improve quality of announcements and make the live miserable to a potential burglar. They will be installed in the kitchen and master room. The only problem is that I cannot control the volume of spoken voices (via OUT1) using rules. I already have enough power resistors and relays to make an attenuator for the night announcements, specially important for the speaker installed in the master room.