When the imaam supposed to be reading a silent namaaz but can be heard by the musallis

AS SALAAMU ALAYKUM MUFTI SAHEB,
KINDLY ADVISE WITH REGARDS TO THE FOLLOWING:
WITR AFTER ESHA, WHILST THE MUSALLI’S ARE PERFORMING THE SUNNAT AND WITR, OTHER PEOPLE WHO HAVE COMPLETED START READINGA KITAAB ALOUD. IS THIS CORRECT.
SHUKRAN