If this is just outdoor background music, there wont be any real stereo separation or necessary SPL levels. To make it very easy, I would suggest you consider a simple 70volt distributed system. You can buy 70 volt line matching transformers at Rad Shack, Lowe's HD that can be used with anyones speakers or you can but outdoor grade speakers already equipped with transformers.
Using this method, you can have several speakers, not worry about impedance matching, and they can even be zoned for a couple different areas of the yard with a separate volume control for each. The entire speaker array can be fed from a mono amp with a 70 volt output designed for distributed audio systems. There are dozens of brands out there. You then simply feed you audio source into the dedicated outdoor amp in parallel if you like with feeds to the indoor systems. The fidelity is quite good through the amp and transformers and is really limited only by the speaker choice and location.