In the state of Ohio an I.E.P. or Individual Education Plan is used to provide specialized education services. The 504 plan is used to provide accomodations when specialized eduction is not required.

If a student is on an IEP - then there is not a 504 plan also, all accomodations are specified within the IEP.

Auditory Processing is big - in the State of Ohio that would most likely need an IEP. Most likely a student would need specialized eduction geared towards learning coping skills for dealing with Auditory Processing as well as specialized education to teach the lesson in a different way than the standard curriculum.

Either way - students with needs are accomodated.