The Arizal says that the Kohen Gadol did not actually utter Hashem’s Ineffable Name. He merely opened his mouth, and the Ineffable Name emanated from him; it was the Shechinah speaking through his mouth. 

When all the people merited to hear the voice of the Shechinah, they sensed Hashem’s presence. Overcome with Awe, they prostrated themselves out of deep veneration of Hashem.  

~ Avodas Yisrael