Question:  

One rakat of jumuah was completed and suddenly the microphone malfunctioned. The voice of the imam was no longer audible, nor was that of the mukabbir. After a while the sound of the microphone returned, however the imam had completed his salah and was reciting duas. So how will those followers complete their salah who did not hear the imam?

Answer: حامدا ومصليا ومسلما

Those followers will take the ruling of a laahiq. That is, when they realized that the imam has completed his salah, they will complete the rest of the rakat without qirat, and then sit and make salam (like usual). (شامي 1:439)

And Allah knows best.
Hazrat Mufti Ahmad Khanpuri Saheb (May Allah preserve him)
Mahmoodul Fataawaa 1/472