One of the oddities of this universe is the profound effect of music upon us homo sapiens. Consider how music stimulates pleasure on the same level, to one degree or another, as that of food and sex. Of course, food and sex serve a basic purpose in our survival, yet how can that be said of music?
Well, I will leave you to ponder that cosmic question while you consider the other remarkable properties of music below and how they can be intimiately linked to our health:
1) Blood Pressure. One study showed that "rhythmically homogenous" music combined with breathing exercises for a half hour a day reduced systolic and diastolic blood pressure in mildly hypertensive patient by 4 and 3, respectively.  Furthermore, the results persisted for one month. A variety of music was allowed, including classical, Celtic and Indian, but rhythmic consistency was required. Another study of researchers found that patients who selected music to listen to before, during and after an operation had significant decreases in blood pressure when compared with controls.  Still other research on seniors showed a drop of and , respectively in the upper and lower blood pressure readings from listening to just 25 minutes per day!  This is unarguably a profound effect for something that many guys consider a waste of time.
2) Cortisol. Many studies have shown that music can decrease cortisol levels. For example, one interesting study told patients that they needed to undergo surgery which, not too surprisingly, produced a large cortisol increase (50%) that stayed high even an hour later. However, those that listened to music had no greater response than controls after the hour. In other words, music completely eliminated the stress reaction by an hour afterwards!
3. Testosterone. Music alters testosterone levels - higher in females and lower in males - according to one study. 
4. Pain Management. Music lowers pain significantly and in some dramatically according to one study of cancer patients.  (Of course, some music can cause pain and it's usually playing in someone car next to me at about 140 dB.) Research has even shown that it could be used as an effective therapy in managing arthritis. 
Music therapy has also had considerable success and one well-known researcher, Alfred Tomatis, is quoted in the book Music: Physician for Times to Come stating that music is literally brain food. He states that "a hidden but primary function of the ear...is to charge the brain with electrical potential. Sounds, especially the ones we make ourselves as singers and speakers, are a fantastic energy food." 
There is even one physicist who claimed that he could correspond musical pitch with the frequencies of the plant's amino acid synthesis and thus accelerate growth significantly.  And, of course, many traditions from the Bible to Robert Monroe have even stated that there is a spiritual power behind music itself. Whatever is going on, the power of music to heal and transform is amazing indeed.
1) Modesti PA, et al "Daily sessions of music can reduce 24-hour ambulatory blood pressure in mild hypertension" ASH Meeting 2008; Abstract 230.
2) Psychosomatic Medicine, 2001, 63:487-492, "Normalization of Hypertensive Responses During Ambulatory Surgical Stress by Perioperative Music"
3) Medicine and Biology Society, 2007, 29th Annual International Conference of the IEEE, "The Effect of Music on Hypertensive Patients"
4) Exp Clin Endocrinol Diabetes, 1994, 102(2):118-120, "Effects of music treatment on salivary cortisol in patients exposed to pre-surgical stress"
5) Neuro Endocrinol Lett, 2003 Jun-Aug, 24(3-4):173-80, "The effects of music and visual stress on testosterone and cortisol in men and women"
6) Oncol Nurs Forum, 1991 Nov-Dec, 18(8):1327-37, "The therapeutic use of music for cancer-related pain"
7) Journal of Advanced Nursing, Dec 2003, 44(5):517 524, "Effect of music on chronic osteoarthritis pain in older people"
8) Music: Physician for Times to Come, Don Campbell, p. 11