The speaker plays the accompaniment track (if the song playing has one).  The accompaniment can be anything from vocals to other instruments.

The library had songs from Elton John and Billy Joel which where the full recorded songs with the piano tracks removed and reproduced live by the self playing piano.

For example, ff you were listening to "Crocodile Rock" over the Sonos system in another room it's hard to tell that it's coming from the piano.  Through the control app (and also on the mixer I installed) you can change the balance between piano & accompaniment track.  I balanced it so the piano was a little more prominent in the mix.