יהיו לרצון אמרי פי

יהיו לרצון אמרי פי

The Rema (O.Ch. 123:6) writes, 

The Chazan at the end of חזרת הש"ץ

does not say.יהיו לרצון אמרי פי

The Mishna V'rurah says the reason

is because he relies on the תתקבל

of the Kaddish, he will be saying 

after ובא לציון.(not to be repetitive)  

The M.V. also quotes the של"ה and

the גאון who argue and hold he should

say it.

The Minhag is to say it quietly, not to

offend the Psak of the Rema.

*Possibly the same reasoning would

apply during Selichos at שמע קולינו.

Since the Chazzan ends the Selichos

with the Kaddish w תתקבל. It would 

sound repetitive saying both יהו לרצון

& תתקבל, therefore, they instituted to 

say יהיו לרצון quietly.