Fix singig all visible songs.

Vocaluxe/Screens/CScreenSong.cs

@@ -543,7 +543,7 @@ namespace Vocaluxe.Screens
                 int SongNr = IDs[CGame.Rand.Next(IDs.Count)];
 
                 EGameMode gm = EGameMode.TR_GAMEMODE_NORMAL;
-                if (CSongs.VisibleSongs[SongNr].IsDuet)
+                if (CSongs.AllSongs[SongNr].IsDuet)
                     gm = EGameMode.TR_GAMEMODE_DUET;
 
                 CGame.AddSong(SongNr, gm);