I have thought about that a lot - not only about songs but movies too. While, like Happiness, I don't have the answer to this, I can relate to her experiences (manifesting things sometimes incredibly accurate when compared to the lyrics of a song I've been listening to). Good and bad. I recognized this pattern long before I even heard of The LoA. And then, once you recognize the pattern, you install the belief that this actually happens and then, guess what? - it does! Even faster, because there's an underlying belief about it.
The way I see it is that, if every thought is creative, if we pay attention to the lyrics in a song (whether we sing along or not) we are in the creative process just as much as if we were simply thinking about something as we usually do. Add emotion to it and things amplify. And songs - and movies, for that matter - are designed to trigger/cause emotions. And many times we relate to some lyrics, bringing back memories associated with the song or the lyrics themselves, in which cases not only are we being affected by the emotion caused by the melody, but we are also remembering the past (past events - that most likely have emotions attached to them as well). So, even if the lyrics are positive, but the song itself triggers a bad memory, you are thinking about it. And, of course, if all of this is true, it's only reasonable to assume that this logic applies both ways - for good or bad memories and emotions and positive/happy or sad lyrics and melodies.
So, right now, in general, I consciously avoid songs with sad lyrics and songs that trigger bad emotions - be it because they are attached to a memory or because the melody makes me feel sad, etc... I carefully choose what I listen to and watch and seek for things that trigger good emotions and thoughts. In fact, I have been compiling a list of songs with this in mind and putting them together in a playlist. I already have a list of songs that most probaby weren't written with the LoA in mind, but in which the lyrics reflect the principle in some way. I've been meaning to start a thread about this so that people can share other songs like these.
So, there's no way to know for sure, but this is my opinion based on my experiences.
But keep in mind that part of this is probably caused because I believe that it happens this way.
