Not sure if it's possible to change sound of horses (for better). But sometimes when horses come behind me, I really can't hear them well over other sounds. Sometimes it feels like horses come and sneak behind and first time you hear something is when you are hit.
It doesn't matter how you try to "balance" (nerf) cav. People lacking awareness will always whine about them. So maybe increasing changes for people to notice them coming helps?
And... there's really nothing wrong with them atm. Higher upkeep is just taking fun away from new players and older don't care if they lose couple k:s more from their pile of millions.