Pretty sure that's a viral infection and not the abx/herx. A family member of mine had the same many years ago (a long time before covid) and her hearing has never been the same, nor has her smell.
It's also documented:
Hearing loss caused by viruses can be mild or severe to profound, unilateral or bilateral. Mechanisms involved in the induction of hearing loss by different viruses vary greatly, ranging from direct damage to inner ear structures, including inner ear hair cells and organ of Corti (as seen in some of the classically described causes of viral hearing loss such as measles), to induction of host immune-mediated damage (Table 2). Following infections with certain viruses,
hearing loss can be reversed or limited by appropriate antiviral therapy.I put the last part in bold because your mom might have a good shot at reversing it still. If it were me I would treat it sufficiently now so that her hearing can still recover.
https://www.ncbi.nlm.nih.gov/pmc/articles/pmc4222184/