Question: Is it okay to recite Kaddish silently?
Answer: Rav Samson Raphael Hirsch (Responsa She’mesh Marpeh, siman 5) writes that reciting Kaddish quietly defeats its purpose, which is to publicly sanctify G-d’s Holy Name. Kaddish is a call to the community to sanctify the name of G-d, which simply does not occur when Kaddish is said silently.
Rav Hirsch’s reasoning makes sense since we know that one may not recite Kaddish without a minyan. This fact suggests that there is an interrelationship between the mourner and the members of the minyan. The mourner, by reciting Kaddish, beckons and invites the community to publicly sanctify G-d’s Name.
This concept harmonizes well with the statement of the Aruch HaShulchan (Orach Chayim 55:1) that the Anshei Knesset Ha’Gedolah ordained the recitation of Kaddish – a public sanctification of G-d’s name – in reaction to the public desecration of G-d’s Holy Name due to the destruction of the Beit HaMikdash. Kaddish, therefore, must be said out loud since only in that manner does it serve as a public sanctification.
The Gemara (Shabbat 119b) states: Whoever says Kaddish with all his strength “opens for himself the gates of the Garden of Eden.”