Is Talking during Khutbah Permissible?
Question

I go to Jumu`ah prayer, but every time I enter the prayer-hall of the mosque, I say Salam and the other worshippers return the greeting, even those who are reading the Quran. When the Khutbah has begun, some worshippers come in and say Salam, and the Imam returns the greeting in a low voice. Is this permissible?
Praise be to Allah.Is talking during the Khutbah permissible?
Those who attend Jumu`ah prayer should be quiet and listen attentively to the Imam when he is preaching. It is not permissible to speak to others, even if that is to tell them to be quiet. Whoever does that has engaged in idle talk, and whoever engages in idle talk, his Jumu`ah prayer does not count. 
Abu Hurayrah (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) said: “If you say to your companion when the Imam is preaching on Friday, ‘Be quiet and listen,’ you have engaged in idle talk.” (Narrated by Al-Bukhari, 892 and Muslim, 851)
This prohibition also applies to responding to a question about Islam, let alone any other kind of speech that has to do with worldly matters. 
Abu Ad-Darda’ (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) sat on the pulpit and addressed the people, and he recited a verse. Ubayy ibn Ka`b was next to me, so I said to him: “O Ubayy, when was this verse revealed?” But he refused to speak to me, so I asked him again and he refused to speak to me, until the Prophet (peace and blessings of Allah be upon him) came down (from the pulpit). Then Ubayy said to me: “You have gained nothing from your Jumu`ah except idle talk.” When the Messenger of Allah (peace and blessings of Allah be upon him) had finished (the prayer), I went to him and told him (what had happened). He said: “Ubayy was right. When you hear your Imam speaking, then keep quiet and listen attentively until he has finished.” (Narrated by Ahmad, 20780 and Ibn Majah, 1111; classed as authentic by Al-Busayri and Al-Albani in Tamam Al-Minnah, p. 338) 
This indicates that it is obligatory to remain silent and listen attentively, and that it is forbidden to speak while the Imam is delivering the Khutbah on Friday. 
Ibn ‘Abd Al-Barr (may Allah have mercy on him) said: “There is no dispute among the scholars of all regions that it is obligatory to remain silent and listen attentively to the Khutbah, for those who hear it.” (Al-Istidhkar, 5/43) 
Some of them held an odd opinion and said that it is not obligatory, but they have no evidence to support their view. 
Ibn Rushd (may Allah have mercy on him) said, concerning the ruling on remaining silent and listening attentively to the Khutbah: 
“With regard to those who do not regard it as obligatory, I do not know that they have any argument except that the suggestion that it is obligatory may seem to contradict the verse (interpretation of the meaning): “So, when the Qur’an is recited, listen to it, and be silent” [Al-A`raf 7:204] – i.e., that it is not obligatory to remain silent for anything except the Quran. There is some weakness in this argument, and Allah knows best. It is very likely that this Hadith did not reach them.” (Bidayat Al-Mujtahid, 1/389) 
When can you talk during the Khutbah?
An exception is made from that in the case of speaking to the Imam, or the Imam speaking to the worshippers for some important reason. 
Anas ibn Malik (may Allah be pleased with him) said: The people were afflicted with a drought at the time of the Prophet (peace and blessings of Allah be upon him), and whilst the Prophet (peace and blessings of Allah be upon him) was delivering the Khutbah one Friday, a Bedouin stood up and said: “O Messenger of Allah, our wealth has been destroyed and our children are starving. Pray to Allah for us.” So he raised his hands [and made Du’a’]. And it rained that day and the next and the next and the next, until the following Friday, when that Bedouin – or he said, someone else – stood up and said: “O Messenger of Allah, our houses have been destroyed and our wealth drowned. Pray to Allah for us.” So he raised his hands… (Narrated by Al-Bukhari, 891and Muslim, 897) 
Jabir ibn ‘Abdullah (may Allah be pleased with him) said: A man came whilst the Prophet (peace and blessings of Allah be upon him) was addressing the people on Friday, and he [the Prophet (peace and blessings of Allah be upon him)] said: “Have you prayed, O So and so?” He said, “No.” He said: “Stand up and pray two Rak’ahs.” (Narrated by Al-Bukhari, 888 and Muslim, 875) 
Whoever quotes these Hadits as evidence that it is permissible for worshippers to speak to one another and that it is not obligatory to remain silent, is not correct.  
Ibn Qudamah (may Allah have mercy on him) said: 
“What they quote as evidence may be understood as referring only to the one who speaks to the Imam or to whom the Imam speaks, because that does not distract anyone from listening to his Khutbah. Hence the Prophet (peace and blessings of Allah be upon him) asked him, “Have you prayed?” and he replied; and ‘Umar (may Allah be pleased with him) asked ‘Uthman (may Allah be pleased with him) a question, when he came in whilst he (`Umar) was preaching, and `Uthman answered. Thus we should understand these reports in this manner, in order to reconcile the reports. It is not correct to draw an analogy with anything else, because when the Imam speaks, he interrupts his Khutbah, so he is not talking to the other person during the Khutbah as such, unlike when others speak during the Khutbah.” (Al-Mughni, 2/85) 
Can you say Yarhamuk-Allah and return Salam during the Khutbah?
With regard to saying Yarhamuk-Allah (may Allah have mercy on you) to one who sneezes or returning Salam whilst the Imam is delivering the Khutbah, the scholars differed concerning that. 
Al-Tirmidhi said in his Sunan, following the Hadith of Abu Hurayrah (may Allah be pleased with him), “If you say to your companion…”: 
They differed concerning returning Salam and saying Yarhamuk-Allah (may Allah have mercy on you) to one who sneezes. Some scholars granted a concession allowing returning Salam and saying Yarhamuk-Allah (may Allah have mercy on him) to one who sneezes whilst the Imam is delivering the Khutbah . This is the view of Ahmad and Ishaq (may Allah have mercy on them). But some of the scholars among the Tabi’in and others regarded that as disliked. This is the view of Al-Shafa’i (may Allah have mercy on him). 
It says in Fatawa Al-Lajnah Ad-Da’imah (8/242): 
“It is not permissible to say Yarhamuk-Allah (may Allah have mercy on you) to one who sneezes or to return Salam whilst the Imam is delivering the Khutbah , according to the correct scholarly opinion, because both of them involve speaking, which is forbidden when the Imam is delivering the Khutbah, because of the general meaning of the Hadith.” 
It also says (8/243): 
“It is not permissible for the one who enters whilst the Imam is delivering the Khutbah on Friday , if he can hear the Khutbah, to greet the people in the mosque, and those in the mosque should not return his greeting whilst the Imam is delivering the Khutbah.” 
And it says (8/244): 
“It is not permissible to speak whilst the khatib is delivering the Friday sermon except for one who speaks to the Imam for an important reason.” 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“It is prohibited to give Salam during the Friday Khutbah, so it is not permissible for one who enters the mosque whilst the Imam is delivering the Khutbah to give Salam, and it is also prohibited to return the greeting.” (Fatawa Ibn ‘Uthaymin, 16/100) 
Shaykh Al-Albani (may Allah have mercy on him) said: 
“If someone says “Be quiet!”, this is not regarded as idle talk from a linguistic point of view, because it comes under the heading of enjoining what is good and forbidding what is evil. However the Prophet (peace and blessings of Allah be upon him) called it idle talk that is not permissible. This comes under the heading of giving precedence to what is more important (namely being silent and listening to the preaching of the khatib) over what is important (namely enjoining what is good during the Khutbah). As this is the case, everything that comes under the heading of enjoining what is good comes under the same ruling as enjoining what is good – so how about if it is of less importance than that? Undoubtedly in that case it is more appropriate that it not be allowed and it comes under the heading of idle talk.” (Al-Ajwibah An-Nafi’ah, p. 45) 
In conclusion, those who attend Jumu`ah prayers have to be silent and listen attentively to the Imam. It is not permissible to speak whilst the Imam is delivering the Khutbah , except in the cases indicated by the evidence, such as speaking to the khatib or responding to him, or in cases of necessity such as saving a blind man from falling and so on. 
Greeting the Imam with Salam or returning the greeting of one who comes in also comes under this prohibition, because there is no concession allowing one to speak with the Imam except in cases of need, which does not include giving or returning greetings. 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in Ash-Sharh Al-Mumti` (5/140): 
"It is not permissible for the Imam to say anything that serves no purpose, rather it should be for a reason connected with the prayer or other suitable topics. But if the Imam says something that serves no purpose, that is not permissible. 
If there is a need, then it is permissible. For example, if a listener does not understand something that the khatib says in his Khutbah, or if the khatib makes a mistake in reciting a verse that alters the meaning, or if he omits a phrase from a verse, and so on.  
Other reasons in which the Imam may speak for a purpose include problems with the loudspeakers, in which the Imam may speak to the engineer and ask him to see what is wrong." 
And Allah knows best.