Does sound, once produced, disappear? Or is the distance getting smaller? If it doesn’t fade, can a sound somehow return to the hearing aid years later?

First, understand what the sound is. Sound is nothing but waves generated in a medium (air or liquid). If a rock is dropped in a pond, waves are created in the water, so sound waves are created when the medium vibrates. Just as waves produced in water to drop rocks weaken as they approach the surface, so waves become weaker (increasing in wavelength) as they move away from the source of vibration. As a result, it ceases to exist after a long distance. That is, a sound or vibration produced now has no chance of returning many years later unless the wave producing the sound gains some energy midway.