Question: What are the halachos of reminding others to say yaaleh veyavo on rosh chodesh? Is it okay to say ‘yaaleh veyavo’ loudly during shemoneh esrei?
Answer: The Shulchan Aruch (OC 236:2) writes that the chazan announces ‘rosh chodesh’ before shemoneh esrei to remind everyone to say yaaleh veyavo. As this is necessary for davening, it is not considered to be an unnecessary hefsek (See Shut Harashba 1:293). The Mishna Berura (Shaar Hatziyun 236:4) points out that one may not announce this during shacharis, though, as there mustn’t be the slightest interruption before shemoneh esrei. Nowadays, the minhag is to announce ‘yaaleh veyavo’, though some bang on a table instead.
The Kaf Hachaim (OC 236:16) writes that the chazzan may announce the words ‘yaaleh veyavo’ loudly while saying his shemoneh esrei. R’ Shlomo Zalman Auerbach (Halichos Shlomo, Rosh Chodesh 1:1) wrote strongly against anyone other than the chazzan / gabbai calling out any words during shemoneh esrei. One is allowed to hint to someone else to say yaaleh veyavo, however. R' Moshe Stern (Be’er Moshe 4:10) writes similarly that while anyone may say the words ‘yaaleh veyavo’ loudly, no one else should do so afterwards.
The Kaf Hachaim (OC 236:17) write that one may not make other announcements during maariv that are of lesser importance, such as al hanissim, though the Mishna Berura (236:7) allows it (See Magen Avraham 236:1).