How would you treat newly diagnosed good risk stage III seminoma with hearing loss?

In order to treat a newly diagnosed stage 3 seminoma in a hearing-impaired patient, my first step is to have the pathology confirmed by an expert GU-tumor pathologist to ensure that this is not actually NSGCT nor large cell lymphoma. While that is in process, I ensure that baseline tumor markers have been checked and particularly that AFP is not elevated (which usually connotes NSGCT).

If this is truly metastatic seminoma, I am quite comfortable replacing cisplatin in BEP with carboplatin, which...

I do not think there is an alternative to cisplatin-based chemotherapy, and BEP would be a very strong preference because it reduces the cumulative dose of cisplatin. It is far from automatic that cisplatin will worsen his hearing. I would pay careful attention to hydration and dehydration during delivery.