Here is an OSHA interpretive letter addressing this question.
OSHA's Noise standard requires only that audiometric testing be made available to all employees whose exposures equal or exceed an 8-hour time-weighted average of 85 dBA. On the other hand, the standard does not prohibit an employer from having a company rule that employees submit to audiometric testing. You should be aware, however, that the Americans With Disabilities Act (ADA) places certain limitations on employer required medical examinations. Among other things, such examinations must be shown to be job related and consistent with business necessity. For guidance on the requirements of the ADA, you may want to contact the Equal Employment Opportunity Commission (EEOC).