Actually, they prefer total silence, but they prefer Enya over ‘dance like a monkey‘ by the New York Dolls. I know, I'm devastated by this also.
Being a monkey means you prefer to find a nice quiet spot where you can suck your thumb and eat some peanuts in peace, apparently. Researchers at MIT and Harvard conducted studies showing that monkeys prefer silence to music, and if they had to choose a music they’d be more inclined to go with Enya over other choices. I know, I was devastated, too.
The end of the above-linked article contained a link to an online survey by MIT researchers dealing with humans’ preferences to certain sounds, chords, and music.