Why should I wear hearing protection when I’m off the clock?

Workplace noise isn’t any more or less harmful than any other type of noise. It’s exactly the same.

Answer Extra: 

Hazardous noise occurs both on the job and off. You are exposed to dangerous noise levels in many areas of your life, whether at a music concert, a racetrack, using a belt sander or just mowing the lawn. All hazardous noise, wherever you are exposed to it, forms part of an overall cumulative risk that can damage your hearing—even if you follow every HPD rule at work.

source: 
source: National Institute of Health Pub. No. 00-4848