Speaking during chazarat hashatz (the repetition of the amidah)

Hi,

One must conduct himself during the repetition of the amidah (shemoneh esreih) as though he was fulfilling his personal obligation to pray with the chazan's recitation. Therefore, one may not speak even when the chazan is silent.

One also may not speak during recitation of added piyutim (poems), inlcuding those recited by everyone and those recited exclusively by the chazan.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 5:20:9)

Have a great day,
Mordechai