When reciting Kaddish together with another person, must one recite Kaddish aloud for the congregation to hear?
Some Poskim rule it is not necessary for one to recite Kaddish out loud if another person is saying the Kaddish loudly. Other Poskim however rule it is necessary for every person saying Kaddish to say it out loud so the congregation can hear. Practically, one is to be stringent in this matter.
 Rav Poalim 2/14 and Ben Ish Chaiy Vayechi 16 in name of Pesach Dvir 125 and Rav Eliyahu Mani that so is custom of Beis Kiel community of Jerusalem, that only the Chazan recites the Kaddish aloud, while the other Chiyuvim do so quietly in order not to confuse his concentration; Kaf Hachaim 55/31-32
 Binyan Tziyon 122, brought in Rav Poalim ibid; Betzeil Hachochmah 5/135 in name of Sefer Hamachkim; Mishnas Sachir 1/11 based on Radbaz 3/446 and 474 that the greater the sound of Kaddish the better; Piskeiy Teshuvos 132/22
 Ben Ish Chaiy Vayechi 16; Rav Poalim ibid