Do the deceased hear their visitors and what they say to them, and is there a way to make them hear if they are unable to?

The basic principle is that the deceased do not hear the speech of the living, based on the Almighty's saying: (Nor can you make those in the graves hear you) and (Indeed, you will not make the dead hear). What happened with the Prophet, peace and blessings be upon him, after Badr was a special case.

The most important things that benefit the deceased after their death are supplication for their forgiveness and mercy. The supplication of a righteous child benefits the parent, just as seeking forgiveness for the deceased elevates their rank in Paradise. Among the deeds that reach the deceased are giving charity on their behalf, performing Hajj and Umrah on their behalf after the living person has fulfilled their own obligatory Hajj, and observing fasts and vows on their behalf. Also, including them in the Udhiyah (sacrifice) and settling their debts reach them.

Therefore, focusing on what benefits the deceased is more important than thinking about whether they hear the voice of the living, while being cautious of innovations such as the fortieth day remembrance.