In the repetition of the Amida, it seems (see Teshuvos Igros Moshe 3 (9), OC 104 (7), OC 125 Biur Halocho d.h. "elo shoskin ..") that the Chazzan should say the pesukim of Kedusho loud enough so that someone who has not yet finished his own Amida can hear them and thereby fulfill his obligation. But some Chazzanim do not, especially on Shabbos and Yom Tov. What sources do they have to rely on?
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In Rambam's text of the siddur (printed after the various laws of prayer, blessings, and the like, in the back of his code), he writes:
I once heard (I do not remember from who) that on Shabbos and Yom Tov since the Davening is done at a slower pace - in general people are holding along with the Chazan - and thus it is unecessary to say the Pesukim out loud.
It is also very possible that the Chazan is saying it out loud, however as there are often more people in Shul on Shabbos/Yom Tov and often they are Davening louder than during the week, it may be hard to hear the Chazzan saying the Pesukim.