I seem to remember another study reported in Equus that indicated that horses do better with no music than they do with constant background music.
I'm with the horses, the staccato tempo that's often found in jazz and rock would not be very soothing if you're stuck in a 12 x 12 stall. They probably feel the way I do when I'm trapped in the waiting area at the doctor's office with the TV playing on some morning news program.
I'd love to know how much they spent on that study. Years ago, somebody did a similar study to determine whether horses preferred to have the barn lights on or off. They figured out the horses didn't really care very much either way, and it cost something like two million dollars.
We used to listen to Top 40/Hip-Hop at my old barn, until that guy Bubba Sparxxx came out with that song that was just him going "booty booty booty booty booty" over and over again (not sure what the song was called - Booty, maybe?). Barn manager had a FIT and said she refused to listen to that garbage anymore and started putting opera and classical on instead.
I don't know if it relaxed the horses, but it did create a very pleasant atmosphere.
Agree with hinderella -- previous research indicated that horses NEED quiet time to rest mentally and physically (which seems fairly obvious!) so that ever present barn radio (aside from annoying the heck out of me) was actually correlated with a higher presence of ulcers. The raw data was not posted so I can't say if it was a spurious correlation but it is plausible, especially with horses who may get little to no turnout.
Worked with a girl who blasted talk radio (the controversial kind) thru the barn while she worked and horses were in. I doubt it was soothing for them! I mentioned it and she told me it kept the deamons in her head away! She was serious. I was freaked out. One of my horses seemed to enjoy hispanic music.