In preemployment medical screening, what are the criteria for the
different levels of hearing ability required for different positions ?

Unless you are speaking of a job which likely has specified parameters
in the public domain (e.g., government, police, military), you should
probably pose that question to the H.R. office for the employer at
issue.

There is an interesting article here:

http://www.uslaw.com/library/article/mzapplicanttests.html?area_id=43

A quote from the page linked above:

"...Employers may not require applicants to take a physical
examination until after an offer of employment has been made."

It is my understanding that, under the terms of the federal Americans
with Disabilities Act, employers are no longer permitted to ask job
applicants about medical information or require a physical examination
PRIOR to offering employment. And AFTER employment is offered, an
employer can only ask for a medical examination if it is required of
all employees holding similar jobs. If you're turned down for a job
because of the results of a medical examination, the employer must be
able to prove that it would have been physically impossible for you to
do the work required.

Violations of the ADA should be brought to the attention of the U.S.
Equal Employment Opportunity Commission (EEOC).

The text of the Americans with Disabilities Act is here:

http://www.eeoc.gov/laws/ada.html

Under "Preemployment," note this:

(A) Prohibited examination or inquiry. Except as provided in paragraph
(3), a covered entity shall not conduct a medical examination or make
inquiries of a job applicant as to whether such applicant is an
individual with a disability or as to the nature or severity of such
disability.

I am not an attorney, and my comments should not be construed as legal
advice, but as general guidelines which may assist you in finding
information related to your question.