A Swedish research within Digital humanities called “Streaming heritage” about streaming in general and Spotify more specifically, is starting to get published in various forms. They are also noting the difficulty of pinning down what kind of commodity a business like Spotify are selling as their own statements about this is constantly changing ad has done so over the years.
A open access special issue can be found here with some output from the project so far.
http://www.cultureunbound.ep.liu.se/article.asp?DOI=10.3384/cu.2000.1525.1792146
After one of the researchers reminded the business press that Spotify in the beginning of its journey started out by streaming music they had acquired through piracy (I remember that I in 2008 listened to music from one of my favorite groups that were only released on Cassette and Vinyl in limited numbers), the company pressured the funders of the research project to withdraw the funding, claiming unethical research methods. Of course they failed but anyway.
https://computersweden.idg.se/2.2683/1.690166/spotify-forsokte-stoppa-forskning