Amazon Echo Dot
First off, to give Alexa control over J River, you need to forward a port and create a static IP. I found everything I needed to do to make the possible by following the instructions on this port forwarding website. The instructions made the process easy enough that I was able to get it to work on my first try (photo below from J River website).
In this case, I opened the Airfoil Satellite iOS app that lets me select which Airplay speakers get audio and directed Airfoil to send audio to my living room receiver. It started playing the music in that room instead. (the pic below is from my Airfoil 5 review. As you can see, it works with Bluetooth, as well. The company is also working on Chromecast integration.)
Echo isn't mobile
The Wrap Up
In short, playing anything from your music collection by just telling Alexa to play it, is simply awesome!