Interesting. The two linked articles reach entirely different conclusions! I can see where playing race radio next to race horses would be pretty stressful since presumably they would associate the announcer’s voice with being on the track!
I know people like having the radio on in the indoor arena when it is raining heavily or windy, the idea being to mask the noises outside for spooky horses. Since my horses isn’t reactive to those things, I’m happier with the sound of the rain on the roof!
I dislike having music playing in the barns or in the arena, especially radio with its constants ad breaks and slightly flat acoustics. I play the radio during my long commutes, so my personal association with the radio is being slightly tense driving in traffic, and I want it off when I’m at the barn!
I do like riding to music the times I’ve practised drill team. So maybe if I could choose my music, and have it played on a decent stereo system, I’d like it.
Horses are so attuned to sounds, that I could imagine a wide variety of responses to music, some learned. Absolutely I can believe that horses would learn to be calm for a particular tune, or up for another, and maybe even want to trot in sync.
As far as in the barns, I expect it would depend on what the larger environment was like. I could see playing music softly as white noise. But if the environment was peaceful and quiet, I’d leave it that way. On the other hand, I haven’t had any adult experience with horses being stalled 24/7, so that might change what you needed to do as far as environment.