1. Hear song.
2. Remember instance of song (memory).
3. Recall emotions related to instance.
4. Experience emotions (mood).
I also stated that the brain's association between music and memory is tight-knit.
It's a bit ridiculous to talk about any art form in an objective manner. Paintings are ultimately what the subjective consumer makes of it. Music is no different.
There is a degree of objectivity available though. Any form of art has an intended mood, regardless of what the consumer ultimately makes of it. This is what the artist sees in it. Through this intention we can take an objective look at music (and furthermore, any artform).
"But Zac, doesn't intention imply subjectivity on the artists behalf?" Right you are! The intended mood of a piece is a subjective affair, but the how of it isn't.
To further clarify, there are objective "tools" that an artist uses to get his subjective "point" across. There are tools to write a sad song contrary to those for a happy song.
To double-further clarify, these tools are actually a product of Enculturation. Specifically, in western music, sad music has a specific sound because we have always used that specific sound for sad music. In another culture, their version of sad music could use different tools than ours (this is a very rudimentary explanation of the concept, as I am not completely familiar with it. I will do some research and post more in-depth on this phenomenon in the future).
"But Zac, that means our "tools" are subjective as well!" You are on the ball today. Yes, the western idea of a "sad" song is subjective relative to other cultures, but for the sake of my argument today, I will be making the assumption I am speaking to someone familiar with western tools.
So, I am going to stop typing this introduction, and go make some music....
....And I'm back. Here are two songs. One is "happy" the other is "sad." Can you figure out which is which?
Mood Music 1 by Project Panda Music
Mood Music 2 by Project Panda Music
I won't spoil which one is which, but I'd bet money you figured it out (not really, I'm in college).