The problem is the fact that Invoke is declared such that it takes a delegate and then a paramarray of arguments for the method being invoked. Your string array is being interpreted as multiple individual arguments rather than a single array argument. Try casting the array as type Object to force it to be interpreted as a single argument:
Code:
this.rtbSongs.Invoke(new SetRtbSongsDelegate(this.SetRtbSongs), (object)newLines);