While it's not clear how much music will be available, it is reportedly ready to pay "a few" labels per stream, no matter how many ads are shown next to those songs.
Amazon does not comment on the rumor. With that statement, it's easy to see that the company offers a free service. Although the existing paid services (Prime Music and Music Unlimited) with a total of 20 million subscribers from 2018 are relatively widespread, it is unclear how many of these people actually listen. Prime Music is more of a bonus than something the listener is looking for. Even if a free service did not have a larger catalog than Prime Music, it would have a much larger potential audience and would make Amazon more competitive than selling more devices with Alexa.