The sample in documentation not only list all music albums but also provide the number of songs in the album and the total lenght in seconds. It should be simplified the query or updated the description accordingly

Text in documentation above:
List all music albums

              $ tracker sparql -q "SELECT ?album ?title COUNT(?song)
                                   AS songs
                                   SUM(?length) AS totallength
                                   WHERE {
                    ?album a nmm:MusicAlbum ;
                    nie:title ?title .
                    ?song nmm:musicAlbum ?album ;
                    nfo:duration ?length
                } GROUP BY ?album"