From what I can remember, the monk she learned it from was mute. How then, did she know how to pronounce the name?
Apoorv020 had it right. It was NOT explained. It was supernatural.
Silence filled the Holiest of Holies.
No stir, no echo, no whisper ofsound.
Even the flames stood silent and motionless in the golden lamp-stands.
And in the deafening silence . . .
Tongueless and unvoiced, the priest spoke the unpronounceable
Name of God.
There was something supernatural going on there. Don't exactly remember what it was, but I think that the mute monk opens his mouth and while Phedre hears the Name of God, nobody else hears anything.