Late last month I purchased a couple Chromecasts for audio playback thru the house. Worked great with Spotify. I could throw a group together and have music everywhere I had a unit and speakers. I was a little disappointed because I have a ton of recordings from the internet and the radio that I would love to cast but there was no way to play my iTunes lib thru Chromcast. I went back to an old program I had called Airfoil. When my old Airports were working, I could stream anything to my speakers, not just iTunes. So I downloaded the latest version of Airfoil and voila, I can now cast anything that plays on my computer to my Chromecasts. For many this is not a big deal, for me it is everything! I listen to a bunch of music on the web and this allows me to cast it anywhere I have one of these inexpensive units ($35).
I hope this helps someone out there.