The first thing that comes to mind is the sound that grounds and centers me as much as my very best meditations.
The sound of the footfalls of a trotting horse on the soft sandy footing of a groomed arena.
A soft chuffing sound as each steel shod foot of a 1200 pound animal strikes the ground in a swinging rhythm.
A steady metronome of sound punctuated by an occasional deep breath or a gentle deep exhale down the nostrils. Hypnotic sounds of a horse at work, footfalls, breathing, the soft squeak of leather against leather, a tail swishing flies, a mummered conversation of human to horse.