The chazan must recite the verses “Kadosh,” “Baruch,” and “Yimloch” together with the congregation, in order to say them with a minyan. He must also recite them out loud, so that if someone is in the middle of the Amidah, he will be able to hear them, thereby fulfilling his obligation of Kedushah, for one who hears is like one who answers (Shulchan Aruch 104:7). After reciting the verses, he should be silent until the majority of the congregation concludes saying the connecting passages, and then he recites them aloud.
If the chazan’s voice is not strong enough to be heard by the congregation, he should wait until most of the congregation finish reciting the verse, so that their voices fade slightly, and only then start to recite the verse. That way, on the one hand, everyone will be able to hear him, and on the other hand, since some have not finished reciting the verses of Kedushah, he is still considered to be reciting them in a minyan.
If the congregation is very large, so that the chazan can only make his voice heard after everyone has finished reciting the verse, there are various opinions as to how he should practice. Some maintain that it is crucial that he recites the verses of Kedushah with the congregation and tries to be heard by at least ten people. Others say that it is most important that everyone can hear him; he should not be concerned that he may not be reciting the verses with ten people. Since he is the chazan, when there is a minyan there that can hear him, he is, indeed, considered to be saying them in a minyan. Therefore, he must wait until his voice can be heard by everyone. Both minhagim are valid.
. The Bei’ur Halachah 125 s.v. “Ela” writes that if the chazan starts the verse before the congregation finishes saying it, he is considered to be reciting it in a minyan. However, if even then they will not hear him, the Bei’ur Halachah is uncertain as to whether the chazan may recite the verses after the congregation has finished, for perhaps because he is saying them in order to fulfill the obligation of the people listening, he need not say them with ten people, and it is enough that ten people hear him to be considered reciting them in a minyan. He leaves this matter unresolved. Divrei Yosef 13 writes that the chazan must say the verses with the congregation. However, Beit Yehudah, part 2, 3, writes that we are not strict that the chazan recite the verses with the congregation and that is also implied from Igrot Moshe, Orach Chaim 3:4.
When a person is praying the Amidah and silently stops so as to hear the chazan recite the Kedushah, as explained in Shulchan Aruch 104:7, yet does not succeed in hearing him, the Levushei Mordechai, part 1:17 writes that he should have kavanah to hear a congregant instead. However, many poskim maintain that since the people in the congregation do not have kavanah to fulfill him in his obligation, this is not effective. Therefore, if he cannot hear the chazan, it is best that he continues praying the Amidah. So is written in Kaf HaChaim 104:36 and implied in the Igrot Moshe there.