Amazon's Echo Input offers Alexa for your existing speakers
 

The latest gadget Amazon is shoehorning Alexa into is an audio dongle called the Echo Input. The $34.99 media bridge has line inputs, Bluetooth and a far-field mic. Think of it like a combo Chromecast Audio and Google Home Mini, but without the speak...

https://www.engadget.com/2018/09/20/...-input-dongle/