All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not an obligation on the Mu’aththin (the person calling the ‘Athaan) to repeat the words of the Imaam so that the congregation would hear them during the prayer. However, if the voice of the Imaam is weak due to sickness or else, then it is desirable for one of those led in the prayer - either the Mu’aththin or anyone else – to repeat in a loud voice the Imaam’s words so that other people led in the prayer would hear the words when he moves from position to position, as it was confirmed that the Prophet led the people in the prayer while sitting down due to sickness, and Abu Bakr repeated for others loudly the Takbeer of the Prophet, sallallaahu ‘alayhi wa sallam. The jurists stated that this is desirable.
However, if the voice of the Imaam reaches all the people led in the prayer, then it is not permissible to repeat the Imaam’s words in this case. Shaykh Ibn Taymiyyah when speaking about repeating the words of the Imaam for others for no need, said: “There is no difference of opinion among the scholars that repeating the words of the Imaam without there being any need is not desirable; rather, many scholars stated that this is disliked, and some of them even stated that whoever does so, his prayer is void, and this view was adopted by the School of Maalik, Ahmad and others . However, if there is a need like the people led in prayer being far from the Imaam, or that the voice of the Imaam is weak, then the scholars differed in opinion about this. However, what is known according to the scholars of the School of Imaam Ahmad is that this is permissible in this case, and this is the most correct opinion according to the scholars of the School of Maalik, and I was informed that Imaam Ahmad had abstained from giving a Fatwa on this issue. It should be noted that in case it is permissible and does not invalidate the prayer of the person who does so, it is a condition that it does not cause any deficiency to the obligations of his prayer.”
Allaah Knows best.