Taken from revach:
Shmoneh Esrei must be said silently. When Yaaleh V’Yavo, Al HaNissim, Mashiv HaRuach, need to be added, people often say the first few words very loud in order to remind the others not to forget. Is this proper?
The Tshuvas HaRashba holds the by Maariv the Gabbai may announce Yaaleh V’Yavo after Kaddish before Shmoneh Esrei. The Maharshal argues and holds it is assur to be mafsik. Therefore he advises that the Gabbai should start Shmoneh Esrei earlier than the Tzibbur and when he gets to Yaaleh V’Yavo he should say the words Yaaleh V’Yavo very loud. From here we learn, says the Be’er Moshe (4:10) that it is permissible for anyone, not only the Gabbai, to say the words very loud in middle of Shmoneh Esrei to remind everyone to say Yaaleh V’Yavo.
He does caution that after the first person does it, no one else should do it since the purpose has already been served. The continuous screams of “Yaaleh V’Yavo” will only disturb people’s tefila and turn davening into a big joke