You don't hear much about ototoxicity (pun completely intended), but I think it is fascinating (what???). Hearing is relatively easy to evaluate and can serve as marker of neurological effect for chemicals targetting the developing nervous system (i.e. harm nerve development in general and you may see poor functioning in general, including poor hearing). Like other cells in the body, cells involved in hearing are vulnerable to oxidative stress. Oxidative attack can result in damage to a cell's DNA, which can lead to development of a cancerous cell or, if you are a fetus, in the development of a birth defect. Your body is not completely defenseless against a cancerous cell. Abnormal cells normally self-destruct in a process called "apoptosis". Its when a cell loses the ability to undergo apoptosis that you may have a serious problem.
When you combine something like Arsenic, which is a great creator of oxidative stress, with exposure to loud noise you get more hearing loss than you would if you were not also exposed to Arsenic. Heavy metals, mercury and lead, are classic ototoxicants as are some antibiotics and drugs for erectile dysfunction.
Hearing loss is also seen when environmental chemicals interfere with neurodevelopment. There is some very interesting work being done in this area by Tomas Trnovec (Slovakia) et al. that focuses on hearing deficits in children exposed to PCBs. PCBs were bannned in the US decades ago, but continued to be produced by the Soviet Block. Poor environmental regulation in Eastern Europe has left those countries with some major environmental health problems. Best wishes to all working in this area.