Roman army public speaking

[Epictetus]

Ammianus (XXVI.2) says... '...as he stretched out his arms to command a hearing, a deep murmur arose. The centuries and maniples began to heckle, and the whole body of cohorts obstinately insisted that a second emperor should be nominated... the cries which were heard did not come from a hired claque but expressed the unanimous wish of the whole throng."

... not only can the soldiers hear Valentinian, but he can hear them (or enough of them to work out the tide of their opinion).

...we might better picture the soldiers gathered in a mob before their commander, waving their hands, muttering and even heckling, the ones at the front passing the words of the speech to their comrades at the back, and so on - more like a riotous political rally than an orderly and formal audience?

There was a long history of groups in the circus or theatre chanting to the emperor or other official. It was part of the event, and (apparently) happened at every such gathering. Often these chants were thanks for the games, but they were also chances to express grievances. Some scholars have said this was the only method of free speech under the more dictatorial emperors. Do we have any evidence that this was common for the army to do likewise?