One praying behind the imam misses a rukn (obligatory part) of the prayer
If someone is praying in a congregation and the loud speaker stops working or the person is drowsy, and he lags behind the Imam by one or more obligatory acts (arkaan) of prayer (i.e., the Imam performed it and he could not because of not hearing the Imam’s voice), then when he returns to consciousness or the sound of the speaker returns back, he should complete the missed obligatory acts and then continue with the Imam. This situation has many cases. Take for example the following case: the Imam recited a verse that contains the word of prostration (sajdah) and the people misunderstood it to be an actual verse of prostration while in reality it is not, so when the Imam says takbeer for ruku‘ at the end of the verse and performs ruku‘, some of the followers (especially those towards the rear of the rows) take it to be the takbeer for the prostration of recital, thus going into prostration. When the Imam stands up from the ruku‘ saying “sami‘a Allahu li man hamidah”, they return back (stand up) from their prostration. Thus, they missed the act of ruku‘ and standing up from it. So it is incumbent on them to complete what they missed and then catch up with the Imam. This is because they did not do it intentionally. However, for the one who intentionally lags behind the Imam (e.g., someone who prolongs his prostration to make a long supplication such that he misses the obligatory act which is after the prostration) the majority of the scholars say that the prayer of someone who misses two consecutive obligatory acts of prayer without a valid excuse, is void and he is sinful.1 However, in principle the Imam must be followed as said by the Prophet (peace be upon him):The Imam is selected to be followed; the, do not differ with him. When he makes the takbeer, make the takbeer, when he goes into ruku‘, make ruku‘. When he says “samia‘ Allahu li man hamidah” (Allah hears he who praises Him), say “Rabbanaa lakal Hamd” (O our Lord! To You belongs the Praise). When he goes into sajdah, make sajdah. If he prays sitting, then all should be sitting.
(Sahih Al-Bukhari, No. 689)