I just went through this exact decision process for our new patio area and i ended up with a couple options. I knew i wanted a pair of outdoor speakers and i wanted to be able to wirelessly stream Spotify over them and control the volume from my phone. I came up with 2 options at 2 very different price points that would offer the same.
Option 1: The Sonos route. A Sonos amp would be the heart of this. Spotify direct streaming and powerful integrated amp built into 1 device. Pair with a set of outdoor speakers and you're all set. Sonos amp's aren't cheap though, however, they are premium and give a seamless and easy integration. I liked this route and explored sourcing a refurbished amp but its still a little more expensive than option 2. I believe these amps can drive 4 speakers, but still only 1 zone. If you want general multi zone capabilities, Sonos is the answer but the cost start to rise quickly. This route will cost you upwards of $1000 if you also source expensive speakers.
Option 2: Amazon Echo + solid state amp. Spotify can be streamed to any Amazon Echo device and many folks seem to have a extra kicking around (And if not, the basic puck models are dirt cheap). These have a line level audio out that can be piped into a mini amp to then drive outdoor speakers. This was the route I took since i had a spare Echo Dot and when paired with a 2 channel solid state Fosi amp off amazon, the total cost was very reasonable. This amp plus the Yamaha speakers will absolutely flood our backyard area even on just 50% volume. I dont see a need for any more power from the amp, power supply or speakers that i chose for casual outdoor listening. I was all in for about $200 with this route.
Here's links to some of these items:
Fosi TB10D 2 channel amp ($80)
Echo Dot gen 2
These are no longer available on amazon as they are superseded by the gen 5. I'd suggest sourcing a cheap NOS model from ebay or marketplace for simplicity.
Outdoor speaker cable ($25)
YAMAHA NS-AW390 PAIR Outdoor 6.5" 2-Way Speakers Refurbished ($80, normally $200 MSRP new)
www.accessories4less.com
I bought these Yammy speakers as it was too good to pass up for a 6.5" driver model. Typical outdoor Polk's and Klipsh's cost many more times that amount but wont offer many times more performance or sound for casual outdoor listening. When they arrived I could not tell they were refurbished in any way. Packaging was new, all mounting accessories included and speaker condition was perfect--definitely not "used" in any way. Maybe they are returns?
The only other items you may need are some optional wall brackets for the audio equipment if you dont have a place to set them. Mine are on a wall in the garage adjacent to the patio. You shouldn't need any additional mounting hardware for the speakers since you can screw them to either the wall or the ends of your roof joists under your soffit if you have typical gable roof with vinyl siding and soffits. YMMV if your construction is different.
Hope this helps.