A couple of thoughts from my long-ago audio engineering training in college-
(I'm one of those who went to school for one thing and...)
Make sure the speakers are designed to be outdoors.
I mean the speakers, not just the housings.
Speakers that are meant to be outdoor have coated cones made of (yechh) plastic fiber or coated paper (better), not high density raw paper.
If the cone looks glossy instead of matte, that's a better sign that they're at least semi-outdoor.
There are a lot of people out there selling "outdoor" speakers, in just a plastic case that's designed to be lightly weatherproof, but all the components inside are regular indoor grade stuff. They usually will rot out the speakers in a year or two, because the internals corrode and the humidity isn't good for the speaker cones.
Mount them out of the weather - up under the eave.
If you mount under an eave, you have the possibility of using the bottom side of roof rafters or wall studs.
I'd put a piece of treated 1x (painted to match vinyl) up under the eave or high on the wall.
That way, your speaker mounts can be whatever they need to be, whether that happens to fit on a stud or not.
I know you didn't ask - but since it might be useful for you, and since
others may trip over this thread looking for information:
I don't know if you're doing a homeowner audio system at 8-12 ohms, or a used commercial 24 volt system. If you are using low voltage 24 speakers
then you can use UV rated Ethernet cable as speaker cable.
If you're doing regular 8-12 ohm speakers, make sure your audio amp
can generate the needed current to drive speakers over very long
distances. Most boomboxes can't do that - the tiny low-quality LMxxx series audio amps they use will burn out or at least badly distort fairly quickly under the loading brought about by 8 ohm speakers over really long speaker wire.
A true amplifier from a decent component system will have better amplfier chips than a Walmart special boombox.
If you're running really long distances (greater than 75 feet of wire)
you really should run low voltage.
You can get conversion kits to allow an existing amplifier and speakers to run at 70v or 24v between the equipment.
For more information, check this out:
http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf