Most programs require at least some CSD classes in undergrad, even if it's not your major - things like Principles of Audiology, Speech/Communication Development in Children, Clinical Audiology, etc. (the course names vary at difference schools) so that everyone's on the same page when they start their Au.D program. Also, if you take these classes and do well in them, then having those professors write strong letters of recommendation will really speak to your abilities to succeed as an audiology student.
Other resume-boosters include working with special needs children (I know things like autism therapy look fantastic and you can use it a lot in your personal statement), research, audiologist shadowing, and volunteering. As mentioned above, NSSHLA's great in terms of giving you information by having local audiologists and SLPs speak to the organization and providing volunteer opportunities like Cochlear Kids Day and the like.
Hope this helps!