The Echo is currently not capable of doing this … i.e., it cannot initiate speech or music playback without the wake word and a verbal request from the user. Everyone is hoping this will change.
The Echo supports Bluetooth playback, so perhaps the Hub V2 bluetooth chip will eventually offer this capability.
Meanwhile, there are some complex (?) workarounds that involve running a VLC or other playback process with various speaker options, including Bluetooth connection to your Echo. Give the forum a search for those configurations.