Cisplatin is added here to only boost the effects of radiation (process is called concurrent chemo-radiation), however it does have number of side-effects.

As Christine mentioned, Cisplatin does have high rate of permanent hearing loss and unfortunately there is no way to prevent it like other side-effects. Discuss possibility of having Carboplatin instead along with Docetaxel with your husband's caregiver.

Since your husband will be having radiotherapy, my recommendation is to go for weekly cisplatin so that he can tolerate the treatment and complete it even though toxicity in either way of treatment would be similar.