The following section was copied from an introductory email after purchasing an Echo Input from Amazon.
Connect to your speakers
Echo Input requires an external speaker and connects via the included 3.5 mm audio cable or Bluetooth.
It does not pair to Wi-Fi speakers.
Ensure your speaker is in pairing mode before connecting via Bluetooth. Recommend pairing with certified Bluetooth Speakers. Search compatible speakers.
AMAZON : Help & Customer Services
Click here to see their comprehensive support page with lots of FAQs under the following headings.
Alexa FAQs Alexa Smart Home Alexa Communication
Alexa-enabled Echo devices Amazon Kids on Alexa and Echo Dot Kids
Echo Buds Echo Connect Alexa on Fire Tablets Alexa on Amazon Fire TV Devices