Increase sound volume for hearing impaired persons


As my son absolutely needed to have an Iphone “to be like the others” :disappointed_relieved:,
I am selling an XA2 Single.
A woman interested by the advert asked me an interesting question: is the sound volume loud enough for hearing impaired persons (translation correct?)?
Sure it’s relative. But I wandered:

Is there a way to increase the sound volume over the normal limits?