Roman army public speaking

[Valerian Pertinax]

I would say that the symbolism of the scepter, diadem, purple cloak, etc would certainly play a communicative role for people too far away to hear an "inauguration" speech. With that in mind, I've also hypothesized that the increasing pageantry of Imperial coronations were a way of compensating, so to speak, for the increasingly treacherous ways that an emperor was usually coming to power. To me, a field general who was just promoted by violently offing his old boss would need all the visual and symbolic help he could get to "sell" himself as the rightful emperor.

Regarding the use of pomp and grandeur to compensate for lack of oratorical skills: plausible, but in my opinion probably not the only reason. We have tales of Boudicca and Spartacus giving rousing pre-battle speeches. (Although in fairness we hear of the latter slaying his horse to indicate solidarity with his men, a powerful gesture even for those outside of earshot). To my knowledge neither of these leaders had any formal oratorical training.

Here my knowledge breaks down a bit: do we have any evidence of a late roman emperor giving a coronation speech? If we do, lack of oratorical training wasn't a roadblock to an emperor addressing his subjects. If not, it seems plausible to me that he would deliberately refrain from speaking to convey how "above" he was his subjects to even communicate to them.