IMHO, the Alexa skills store is mostly a way for Amazon have other companies do their product research for them, before they build the features into the platform themselves.
But yeah, I don't blame you for being cautious of their intentions. Developers have been burned in the past on these big platforms (e.g., Twitter [1]), but I have hope that they will realize the pie is bigger if they make their platform attractive to 3rd party devs.
[1] http://nordicapis.com/twitter-10-year-struggle-with-develope...
I read through their SDK, and music playback isn't really supported. You can do clips up to 90 seconds, but they have to be encoded at 48kHz and at a certain bitrate.