The sound of tilling the ground with a hoe is affected by the Friendly Creatures slider in sound options. I'd think it would be under Blocks, or maybe Players or Ambient/Environment, but definitely not Friendly Creatures.
Linked issues
Comments

Confirmed for 1.8.1-pre2.
Confirmed for 1.8.4.
Related to / duplicate of MC-36360. I think it would be better if all the sounds that are wrongly controlled by the Friendly Creatures slider were grouped under the same report.
Confirmed. Sounds of non-hoe tools not affected. Tilling sound of all hoe types are affected by the "Friendly Creatures" volume slider.