In MarshalJSON() for Song a string field Duration was removed, just msec left.

internal/radio/song.go

@@ -34,14 +34,12 @@ func (s *Song) MarshalJSON() ([]byte, error) {
 		Artist       string `json:"artist"`
 		Title        string `json:"title"`
 		DurationMill int64  `json:"duration_msec"`
-		Duration     string `json:"duration"`
 		MaxListeners int    `json:"listeners"`
 		StartAt      string `json:"start_at"`
 	}{
 		Artist:       s.Artist,
 		Title:        s.Title,
 		DurationMill: s.Duration.Milliseconds(),
-		Duration:     s.DurationString(),
 		MaxListeners: s.MaxListeners,
 		StartAt:      s.StartAt.UTC().Format(time.RFC3339)})
 }
@@ -77,13 +75,13 @@ func (sl *SongList) Add(newSong Song) {
 }
 
 // Current returns a currently playing song.
-func (sl *SongList) Current() Song {
+func (sl *SongList) Current() *Song {
 	sl.Lock()
 	defer sl.Unlock()
 	if sl.current.StartAt.Year() == 1 {
-		return Song{}
+		return nil
 	}
-	return sl.current
+	return &sl.current
 }
 
 // UpdateCurrentMaxListeners checks and updates a maximal number of listeners