http://www.techradar.com/news/alexa-has-come-to-mobile-amazon-music-streaming

Amazon has announced that its digital assistant Alexa is now available in the Amazon Music app on iOS and Android devices. At the moment the feature is only available in the UK, US, Germany and Austria. 

If you’re used to using Alexa on an Echo, or Echo Dot, it will be a slightly different experience, as you won’t have access to the full range of Skills (Amazon’s name for its apps) but will allow you to make more natural track selections.