This is avodah applies particularly to a great person who makes incredible yichudim (spiritual unifications) and tikkunim with his speech, as Rebbe Nachman states: “A great person must be very careful to [limit his speech].” Before the shofar blasts, it is not possible to make tikkunim through words, since we must first disconnect the Malchus of Kedushah – that relates to our speech – from the klipos and darkness of the material world. Even though the speech of the tzaddikim is certainly pure of any form of contamination, nonetheless, the overall tikkun of the world at this time is to make a fresh start by throwing away our seichel and speech so that we rebuild them in a new and better form. Thus, the tzaddikim must be even more careful to only speak the words that are necessary for doing mitzvos. This is why Rebbe Nachman says that, even in tefillah, he only recited the very holy piyyutim of R’ Eliezer HaKalir.
To summarize, by limiting our speech on the first day of Rosh Hashanah and being mevatel ourselves to Hashem with pure emunah, we elevate the Malchus of Kedushah and crown Hashem King. Then, after we hear the powerful shofar blasts we receive a new seichel and a clear awareness of Hashem that guides us to speak holy words throughout the entire year, even as we’re involved in the material world.
