Learn Italian: Se l'udito si affievolisce, si deve usare un dispositivo. - If hearing weakens, one must use a device.

You can use the andare + present gerund structure. Since affievolirsi is reflexive, the pronoun si can go before the conjugated verb or attach to the gerund. You would write Se l'udito si va affievolendo... or Se l'udito va affievolendosi... to mean 'If hearing is gradually weakening...'.

Why is there a si before affievolisce?

It belongs to the pronominal verb affievolirsi (to weaken, to grow faint). While affievolire means to weaken something else, adding si makes it intransitive, describing a change of state happening to the subject itself (the hearing).

In si deve usare, is that also a reflexive si?

No, that is the impersonal si, which translates to the generic 'one' or 'you' in English, so si deve usare means 'one must use'. It is very common in Italian to see both the reflexive si and the impersonal si used in the same sentence with completely different functions.

With the modal verb dovere, could we attach the impersonal si to the end of the infinitive, like deve usarsi?