Is it an innovation (bid'ah) or permissible to inform the one being led in prayer (ma'mum)?

The fundamental principle is that the congregant's vocalization is not prescribed, and jurists have explicitly stated its disfavor (makrooh) because it causes distraction to the worshippers. However, if there is a need for it, such as the imam's illness or the worshippers' inability to hear his voice, then it is sunnah for one of the congregants to make the takbeer and tahmeed audible to the people on behalf of the imam, as Abu Bakr did when the Prophet, peace and blessings be upon him, was ill. Thus, its least status is disfavor when there is no need for it.