Scott Adams wrote a great post about music and mood a while ago, but it's since been removed and put in his book.  A cached copy says,

"Sometimes I wonder why music is legal. Music can alter your mood and your body chemistry just like any illegal drug. The fact that it goes into your body through your ear shouldn't make a difference. We take drugs via practically every other hole in our body -- mouth, butt, eyeballs, nose -- you name it. Ain't nothing special about an ear."

It's amazing how true this really is.  Music has a profound effect on mood, energy, creativity, etc.  It truly affects the body like a drug.  When certain songs come on the radio while I'm driving, I tend to drive faster or more aggressively.  Wendy says certain songs on her iPod make her run faster or work out harder.  Certain songs make you relax.  Certain songs make you think.  Certain songs make you feel like Superman.  It's undeniable (at least for me) that music has a direct effect on my mood.