From April of last year:
What I want is to point Alexa at a NAS or other iTunes library (even better my iTunes Match account, but I’m being realistic here) and have her create and curate custom playlists from my library using only voice commands. If I say:
Alexa, play songs like Smells Like Teen Spirit.
I want her to create a Pandora-style playlist on the fly using the songs in my library.
In preparation for the HomePod, Siri has also learned a new music skill: generating a mix of songs you may like based on your listening history from Apple Music. Ask Siri to “play something I like”, and, like a virtual DJ, it will play a personalized station of songs that match your taste and listening habits. As you listen, you can tell Siri which songs you like, so you’ll get more similar ones down the queue.
My example seeded the list with a specific song, because I contain multitudes, but tailoring the list while it’s playing is right on. And of course this is Siri, not Alexa, but progress is progress.