Q. Is it correct for someone to mention aloud during the quiet shemonesre mashiv haruach or yaaleh veyavo to remind others to also say, or is it a hefsek?

A. Shulchan Aruch (O.H. 101: 2) rules that one should not rise his voice when davening, based on Talmud (Brochos 31) as described in the tefila of Chana. Also the one raising his voice is considered to have deficient faith or belong to the false prophets (Brochos 24b, see Mishna Berura ibid. 6).
Adding to the above, Horav Shlomo Miller’s Shlit’a opinion is that it is not an issue of prohibited hefsek, but rather the undesirable praying loudly disturbance of mispalelim trying to concentrate in their prayers. Therefore, it should be avoided and other means to remind the presents of the required additions and changes to the tefila should be used.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a