If you ignore her pretending not to be able to hear what she said what does she do as in repeat it louder which would be the norm or get frustrated and cry/scream/whine or does she just go off and not care?

What does she do at home? Has she been told to "be quiet" too many times for disturbing the adults as in was she always like this or did she used to be talkative and louder?

For sure meeting with a speech pathologist or other professional is probably in order. The good news in a sense is that if she is going off to school that will be picked up immediately and they will get her on the list for assessment and services which will take place through the school - assuming the wait list isn't too long.