There are many verbs in English (verbs of perception for example) which are not used in progressive forms, e.g hear, believe, know, understand, see etc. but in solutions all these verbs are to be used in 'ing' form, which is a mistake for English speakers. So the solution for 'lui sta sentendo quella voce' should be 'he hears that voice' not he is hearing. I hope this will be taken into consideration and the correct answers will not be rejected.
Thanks in advance
While not a grammatical or semantic impossibility, there is something very odd about DL's English translation: I am hearing that voice. It is the auditory version of the similarly odd: I am seeing that object. When speaking of current rather than repeated action, one expects to hear the progressive used with the auditory and visual perceptive verbs listen and watch, because these require sustained attention. The verb look can be either momentary or sustained, so the progressive is not surprising or special. Though we do say such things as 'He is hearing (/seeing) things' to describe a person hallucinating, it is normal to say 'Do you hear (/see) that?', and NOT 'Are you hearing (/seeing) that?'.