Absolutely get some ChromeCast Audio dongles!! Especially if you want to stream from Spotify or Google Music. This is by far the best option for someone who wants to use existing stereos (or multiple stereos for perfect synchronized whole-home audio). Each unit costs about $40 and connects to the back of your stereo (very small, hides out of view). It comes with a special "combo" output jack that can be connected to a Toslink (optical audio cable) or a mini stereo plug. If your stereo is older, you will then just need an old school "RCA to mini stereo" adapter from Lowe's ($5).
Now, here is where ChromeCast shines- Whole Home Audio!!!! All units can be grouped together. I have one unit called Kitchen and another called Basement. I also have assigned both of these units to a "group" called "House Party.". When I stream to "house party" the music plays on both stereos (synched perfectly as if I had a hard-wired whole home audio system). Super cool!!!!
ChromeCast Audio units are connected to your home wifi (similar to Sonos) but unlike Sonos, you won't have to take out a second mortgage for a whole-home audio system.
FYI, don't waste your time with Bluetooth or those toy Bluetooth speakers - unless you love hearing your phone ring through your speakers during a house party. Bluetooth is great for vehicles... and that's about it. At home, extended Bluetooth use will quickly drain your phone's battery, and is a lower quality audio than Chromecast (which has HD audio).
Anyway, try it out, just make sure to ask for ChromeCast AUDIO! Otherwise the pimple face kid at Best Buy will give you the ChromeCast dongle that connects to televisions. The ChromeCast Audio comes with a short yellow audio cable.
Get one now!