If you've bought into the Apple ecosystem the Airport Express is a great little product. It connects to your wifi (or wired) network and has a 3.5mm female audio plug on the back for your audio connection. With that you can play music from any iOS device or any Mac/PC running iTunes via AirPlay. Sound quality is better over wifi/AirPlay than it is over Bluetooth. Just click on the little radiating triangle AirPlay icon and it brings up a list of your AirPlay devices, including your Airport Express. In my case, I have a few around the house and on the pool and patio. You can play to one set of speakers or all. For example, if it's just me swimming laps in the pool, I'll set it to just the pool speakers, but when we have parties for our teenagers we'll select all of AirPlay devices so that we play music on all of the speakers at once.
Think about it as a wireless bridge. If you have digital audio (essentially anything you can play on your computer) that you want to play on speakers other than those on your computer, then the Airport Express creates that bridge. The little Airport Express can do a lot of other stuff, too--but the audio function is what I use most.
I'm sure there's an Android/Google equivalent out there (Chromecast audio?), but this was what was available at the time we set it up and it's worked pretty seamlessly.