Synchronize on activeAudioPlayers when changing them

airsonic-main/src/main/java/org/airsonic/player/service/JukeboxJavaService.java

@@ -255,6 +255,7 @@ public class JukeboxJavaService {
                     return airsonicPlayer.getPlayQueue().getIndex();
                 }
             });
+            synchronized (activeAudioPlayers) {
                 // Close any other player using the same mixer.
                 String mixer = airsonicPlayer.getJavaJukeboxMixer();
                 if (StringUtils.isBlank(mixer)) {
@@ -266,6 +267,7 @@ public class JukeboxJavaService {
                         player.close();
                     }
                 });
+            }
             audioPlayer.play();
         }
     }