Handbook of ROCOR Liturgical Practice:

In what way are the mystical prayers to be read by the Priest?

While it is well known that the mystical prayers are read ‘secretly’ (i.e., not audibly to the faithful in the Nave), and this matter itself requires no further comment, sometimes it is asked precisely how these prayers are to be read. Are they to be whispered? said completely silently / internally?

While there is no absolute rule on this question, and some clerics read the mystical prayers completely silently (that is, inaudibly, reading them and praying internally without any vocalised sound), the oldest practice is for them to be spoken softly by the chief celebrant, in a voice intended solely for himself. This is a helpful way of keeping one’s mind from becoming distracted, which can easily happen when praying wholly internally — his soft speaking of the words calls the Priest’s attention to the phrases he is praying, and keeps his mind and heart focussed on this ministerial work on behalf of the people.

It should be specially noted that when multiple priests concelebrate, only the chief celebrant (i.e. the first priest, or if a Hierarchical service, the Bishop), should say the prayers in this manner; all others should pray the mystical prayers completely silently, so as to prevent a cacophany of voices around the Holy Table.

Under no circumstances are the mystical prayers vocalised audibly for the faithful, as this is not their purpose. But, in a connected way, the exclamations and audible prayers should be chanted clearly and somberly by the Priest, in a loud voice suitable to the size of the church building, so that they can be clearly heard and understood by all the faithful in the Nave.