How about reading up on Psycho Acoustics? The most basic definition is that it is the science of how sounds makes us feel.
http://en.wikipedia.org/wiki/Psychoacoustics
I can relate to your experience having spent a good deal of time trying to figure out the same thing. I noticed it first when listening to certain types of music. Typically for me I get the same effect when music has a tight repetitive pattern that evolves slowly over time, or when the combination of sounds interact a peak points during a piece, and I am able to reproduce those feelings of euphoria nearly every time if the setting is right...some times to the point of tear secretion.
Examples for me would be listening to Jeff Buckley's title track from his album Grace, pretty much anything by Beethoven, certain electronic tracks by Boards of Canada, or live tracks by Godspeed You! Black Emperor, and Silver Mt. Zion.
I can't say these examples would work for every one, however I do know of a lot of people who report similar things when listening to musical or vocal pieces.
As an experiment perhaps you would listen to a piece that I know provides me with this effect and you can see if it does the same for you? If you are interested go to the link and listen to TRACK 3 titled "American Motor Over Smoldered Field"...headphones are a help, or a nice volume level above the mid range point:
http://www.archive.org/details/asmz2005-06-16.mic.flac16
Typically for me, treating it as a sort of meditation helps achieve the effect greatly.
From my experience working in psychiatry and dealing with people who have had various brain ailments such as cancer, I know that the brain can produce some pretty amazing and or terrifying things...to use the brain tumor example...a person I have known reported hearing the sounds of ringing bells, radio static a low levels, and low frequency electrical noises, along with people talking as if in the background or over a radio. In the strictly psychiatric end all manner of auditory hallucinations and somatic sensations have been reported, although most are typically voices, sometimes of those familiar to them, and almost always carrying a negative connotation...like "your evil" or "you fat and ugly" or "kill yourself"....