The Americans with Disabilities Act (ADA) is the federal law that prohibits discrimination against individuals with disabilities. Title I of the ADA covers employment by private employers with 15 or more employees and state and local government employers of the same size. Section 501 of the Rehabilitation Act provides the same protections for federal employees and applicants for federal employment. Most states also have their own laws prohibiting employment discrimination on the basis of disability. Some of these state laws may apply to smaller employers and provide protections in addition to those available under the American Disabilities Act. The U.S. Equal Employment Opportunity Commission (EEOC) enforces the employment provisions of the ADA.
A hearing impairment is a disability under the ADA if: (1) it substantially limits a major life activity; (2) it substantially limited a major life activity in the past; or (3) the employer regarded (or treated) the individual as if his or her hearing impairment was substantially limiting. The determination of whether a hearing impairment is substantially limiting is made on an individualized, case-by-case basis.
Source: Questions and Answers about Deafness and Hearing Impairments in the Workplace and the Americans with Disabilities Act, http://www.eeoc.gov/facts/deafness.html#fn8