Assalamualaikum ustaz. There is an imam who gave the Friday sermon in such a low voice that some of the makmum couldn’t hear the content of the sermon. Is their Friday prayer valid? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Friday sermon is a weekly assembly for all Muslims. Allah SWT states:
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نُودِي لِلصَّلاةِ مِن يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلَى ذِكْرِ الله وَذَرُوا الْبَيْعَ ذَلِكُمْ خَيْرٌ لَّكُمْ إِن كُنتُمْ تَعْلَمُونَ
“O you who have believed, when [the adhan] is called for the prayer on the day of Jumu’ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew.” 
Listening to the Friday sermon is a commandment. Although jurists have differing opinions on whether it is obligatory or sunnah, however, it is a prioritized sanction for those who attend to listen to the sermon attentively.
The valid condition of the Friday sermon is that it must be preceded with two sermons according to the jumhur of scholars according to madhhab al-Syafie. This is as stated by Imam al-Nawawi in his book al-Majmu’ Syarh al-Muhazzab where he states:
“Friday prayer is invalid until it is preceded with two sermons, according to the hadith narrated from the Prophet PBUH, where he said: “Pray as you see me pray.” Friday prayer cannot be performed except it being preceded with the two sermons, where according to a hadith narrated by Ibn Umar RA, he said: “Rasullullah PBUH always deliver the sermon on Fridays and he would sit in between the sermons.” 
Thus, it is clear that the Friday prayer sermon is one of the valid conditions of Friday of which if it is absent then the Friday prayer is invalid. According to Imam Ibn Qudamah, this has been agreed by scholars except for al-Hasan al-Basri. 
Inaudible Friday Sermon
Regarding the above issue, we state the following:
If the sermon is heard by forty people, then the Friday prayer is valid. In the book al-Fiqh al-Manhaji (1/205-206), regarding the conditions of the two sermons, among others it states:
Both the Friday sermons have several conditions and they are:
- When delivering the sermon, the khatib must be standing if he is capable. Between the sermons, it is alternated with the khatib sitting. This is based on the hadith from Jabir bin Samurah RA, he said:
كَانَ النَّبِىُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَخْطُبُ خُطْبَتَيْنِ يَجْلِسُ بَيْنَهُمَا وَكَانَ يَخْطُبُ قَائِمًا
“ “The Prophet used to preach standing, then he would sit down, then stand and preach standing,” 
- The khutbah is not delivered after the Friday prayer. This is to follow the sunnah of Rasullullah PBUH as it is known from the hadiths regarding the Friday prayer and ijma’ of scholars.
- The khatib must be in a pure state and free from unforgivable najis on his clothing, body and place of prayer. The khatib must also have his aurah covered. Furthermore, the two sermons are a replacement for the two rakaat of Zuhur prayer. Hence the conditions for the seron is the same as the conditions for prayer, such as one must be in a pure state and others.
- The requisites of the sermon must be delivered in Arabic. The khatib must deliver all the requisites in Arabic even if the congregants wouldn’t be able to understand them. If none of them can speak Arabic while they have time, then all of the congregants are sinful and they cannot perform the Friday prayer rather they have to perform the Zuhur prayer. If there is no time to learn Arabic, then the requisites of the sermon should be translated to any suitable language and the Friday prayer is considered valid.
- The requisites of the first and second sermon are delivered consecutively and the second sermon and the Friday prayer is succession. If there is a long interval between the first and second sermon or the second sermon and the Friday prayer, then the sermon is considered invalid. If the sermon can be repeated, then it is obligatory for it to be repeated, if not, then one should proceed with the Zuhur prayer.
- The requisites of the sermon must be heard by 40 congregants who fulfil the obligatory conditions of Friday prayer.
The requisites of the two sermons are:
- Praise Allah SWT with any words.
- Praise and salutation for the Prophet PBUH with any praise and salutations. The utterance of the Prophet PBUH’s name must be clear such as the utterance of Prophet, Messenger or Muhammad and using pronouns for them is not enough.
- Advise to always have taqwa to Allah SWT suing any phrase and language style.
These are the three requisites for both the sermons. Hence, both sermons will be deemed invalid except when all of these requisites are fulfilled. Reciting one verse of the Quran in one of the two sermons. The verse of the Quran recited must be understood and its meaning is clear. It is insufficient to recite one verse consisting of several letters at the beginning of a surah. Supplicating for the believers in the second sermon with any supplication.” 
Thus, the Friday prayer is valid if the sermon is heard by 40 resident congregants. If other congregants excluding the 40 congregants couldn’t hear the sermon, then it doesn’t affect the validity of the Friday sermon. Hopefully, we are given understanding in this religion of ours. Amin.