My ex-SIL and I tested this theory back in the late 70's.
We learned our horses (there were four) got all antsy when we put the radio on what was then known as "hard rock".
They were much better behaved and willing, when we left the radio on a country station.
Dad used to keep "elevator music" on for the milk cows, thinking the quieter more peaceful music would help them give more milk.
I also will not keep a radio on at night but, I do turn the radio on, first thing in the morning. If I can't get a country station to come in clear, I am forced to put the radio on Super Talk 99 so there's some noise.
It's really more a routine, as my four know the radio comes on, they get hay, they get fed an hour or so after that, then they get turned out.
While the time frame varies, the routine stays the same, always including the radio