So who knew? Thanks to a tip off from my bookie MJB, I have found out that quite a few race horses wear earplugs when they race!
It makes sense, but was a surprise! Race tracks are loud, very loud. And horses have very good and sensitive hearing. The jockey does not want the horse distracted or scared by crowd noise. As this article notes, a pair of $6 earplugs helped win the Belmont race in 2015. (You can see the earplugs in the photo above.) As the article notes, other trainers and owners are following suit.
Now this year, the races are being run with no audiences, so maybe the earplugs are not necessary. But given that horses can hear sound that is far away, I would think they would use them in every race.