The Unlimited Music Streaming promo simply means that you can stream as much music as you like from compatible services without using up your data allowance @Anonymous, it doesn't include the cost of any music streaming service itself I'm afraid. Although some services, including Spotify, do offer a free package with some limitations.
I work for O2, however any advice or opinion given in this community should be considered my own and not necessarily representative of Telefónica UK Limited.
