The British Army is very strict about hearing protection during training but under combat conditions many soldiers ignore it and have uncovered ears due to the need to listen for the enemy and orders clearly, so sadly hearing damage in soldiers has been very common in recent years (especially in Afghanistan) as Col Snook will be very aware.
I suffer from hearing damage in certain frequencies from being a mortarman despite always using hearing protection so it doesn’t have to be modern high velocity weapons to cause such damage (just loud!!).
Although I haven’t read any of Snooks books it has always struck me as very likely that Lt Bromhead and the men under his command must all have been very likely to suffer hearing damage at Rorke’s Drift due to the extensive firing in a concentrated area and over a much longer period than any other battles of the Zulu War (or indeed other wars come to that).
I’m not aware of any evidence that suggests Bromhead had poor hearing before that battle (please correct me if I’m wrong), so it seems most likely to me that he suffered his hearing damage at Rorkes Drift, and that it’s highly likely he wasn’t alone.