There's really no routing in the M1 of audio, since it only handles one channel.
You have a couple of options... you can partition things out either by using relays to disconnect sets of speakers, or use the Elk800 amplifier and turn it off/on. The amplifier is easier because you'd have two separate volume controls; one for each area... and it wouldn't fluctuate and you wouldn't need to compensate for changing resistance (Ohms) when connecting/disconnecting a set of speakers.
The rest needs to be done via rules... This has me wondering - somewhere Spanky said how to get to the line level audio *before* it gets to the onboard amplifier - i'd wonder if these still have the audio when you use rules to turn off the audio amplifier... meaning you could turn off the amp, turn on the 800, then play your announcement... Otherwise if you want audio in the garage but not the house, you'd also need a relay to disrupt the speakers inside the house. Of course you could just use two 800's and skip the onboard amp then turn either one on/off at will; but I'd prefer a sort of fail-safe mode where when in doubt, the inside speakers do go off in the event of a break-in or alarm condition.
Or if you really wanted to be creative - depending on how many announcements you have, you could also use something like an Elk124 recordable sound module and make up to 8 specific recordings that would play only in the garage when certain events happen - this would be controlled entirely through rules and would never affect the house because it wouldn't even be hooked up in parallel.
The rules could be a slight pain... basically you'll have to use rules to turn the audio in each zone on/off before an event announces; then also trigger an output for a few seconds that, when turned off, turns the audio back on.
Of course, you could just not have any audio in/for the garage zone... I know I've seen stuff like this come up before when people have guest-houses or in-law quarters on the property that they want hooked to the same system.
