Kindly get the following tests done. I assume that an OAE (Oto Acoustic Emission) Test has been done earlier.
1. ASSR (Auditory Steady State Response) Testing
2. Aided Audiometry
3. Aided Behavioral / Play Observation Audiometry
4. Impedence Audiogram to rule out middle ear defects.
5. Genetic Testing
It is important to know that if sound does not reach the brain within the first few years, it may never develop the ability to understand speech due to a process known as Neural Scavenging where that part of the brain reserved for hearing is taken up by the other senses. A hearing aid
is a priority to amplify as much sound as possible till the diagnosis of the exact problem.