I have a radio in my barn, because I fill it is calming for my horse since she is the only horse there, so I fill it helps her too here other voices so she doesn't fill all alone when I'm not there.
I have rescued mustangs and found that singing around them helps to calm them. Not that I'm a great vocalist, and I just sing or hum and it helps them immensely. You can see the reaction quite quickly!
Definately sing while trail riding because the other birds and animals hear us coming and scatter before we come up on them!