Islam pays great attention to the spread of the atmosphere of peace and brotherhood in society. Hence, it makes it highly recommended to start people with Salam.
The Prophet, peace and blessings be upon him, said: “You shall not enter Paradise until you believe, and you will not be true believers until you love one another. Shall I inform you of something that will cause you to love one another? Spread Salam (peace) among yourselves.”
Yet it becomes an obligation to respond to people’s greeting. Almighty Allah says: “When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally.” (An-Nisa’: 86)

Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, states: “In his book al-Adhkar, Imam An-Nawawi reported on the authority of Abi Sad al-Mitwali, that if a person writes the greeting As-Salaamu `alykum in a letter, it is incumbent on the one to whom the letter is delivered to greet him back; and al-Wahidi, in effect, holds the same opinion. This indicates that once the greeting, written or verbal, is voiced to a certain person or a particular group, they are under an obligation to respond. The same ruling applies to the recorded greeting.

As for the greeting mentioned in a recorded message and broadcast on air, in Radio or TV, is it obligatory for the listeners and viewers to return the greeting? And can the audience or viewers be considered as group of people intended for the greeting?
Well, the ruling here is not clear. However, what appears to me, through reviewing the Juristic books of the four renowned Imams, is that if a person hears a recorded verse that requires recital prostration, he/she is not required to prostrate. However, jurists hold incoherent justification for this ruling. Some said, ‘The reason is that the greeting here is not from a human being’; while others said, ‘This is because the recitation by the cassette player is not directed at a specific group.’
So, if we draw analogy here between the recorded greeting and prostration for the Qur’anic recitation, it may be said that responding to such greeting is not required. But I still have reservation regarding this ruling for which I have not found any concrete proof to support.
But what is the ruling when it comes to live “greeting” voiced by the announcer or his guest, or the one mentioned by a writer or a publisher in the book?
It seems that one must respond, since it is voiced intentionally by a person. The reason for this is that TV or Radio as well as books act as channels through which such greeting is transmitted to people, exactly just as the microphone used by people in communication, in a mosque or a wedding ceremony. But if such greeting is recorded on a tape, one should not answer his greeting back in conformity with the aforementioned ruling on recitation prostration.
Some may argue that the announcer, his guest, or publisher do not exchange greetings with real people but with imaginary ones, since they do not see or feel them; accordingly, one should not greet them back.
However, it is obligatory for a person to answer back. Scholars said: “At least he must speak up so that the greeting person can hear him, and if he does not hear him, it is still an individual duty to respond” [This was mentioned by An-Nawawi in his book al-Adhkar” and by al-Mitwali].

In consequence, it is incumbent upon the radio listeners or TV viewers to respond to “Salaam” greeting even if the announcers do not, nay cannot, hear them. In addition, if some people did it, it would not be obligatory for the others to answer back since it is a collective duty. I reckon, nay am indeed sure, that at least one of the thousands or millions of dear viewers will answer that greeting back making it optional for others to do the same.”
Almighty Allah knows best.