amazon echo devices will play most commercial music streaming subscriptions ( not google music though) e.g. Spotify can be set as a default, via the Alexa app, Apple music is also now supported
We use Spotify - no sense paying for an amazon music subscription which has the same music, esp as Spotify premium has a higher bit rate [ not that you'd notice that on a dot ! , but on something like a Sonos One, you maybe would ].
and there is a way { ask google} to add local content into spotify as a playlist which is then accessible from any Device linked to the Spotify account. so you could probably get at any owned or borrowed MP3 audiobooks like that
and Plex are, allegedly, working on making your plex music collection stream directly to echo. plex control for other devices is already supported as an Alexa skill
https://techcrunch.com/2017/02/02/pl...-asking-alexa/