Buy a receiver or mini amp.... attached your speakers to this. Run an AUX or RCA-->3.5mm from the receiver to a Chromecast Audio. Stream the media to the CC-Audio, it will be play from the speakers and cost 1/5th of what Sonos will.
If you're not looking for anything crazy in terms of power I suggest:
This amp -
https://www.parts-express.com/lepai-...pply--310-3000 - its tiny, cheap, and works well
Chromecast Audio -
https://store.google.com/us/product/...audio?hl=en-US - lets you stream via your wifi to the lil' amp
The CC-A is awesome because you can setup "groups" of multiple CC-A devices and have the music sync (like sonos but way cheaper). I have 3x at our place - my basement home theater, living room 2.1 system, and outdoor speakers. I can send my music via to all 3x receivers/amps at the same time so my whole house / beach is covered in sound.