We know from writers such as Suetonius in his Vita Divi Iuli that legionnaires would sing some quite raunchy and bawdy tunes, even during a triumph of their leader.
There were open allusions to sexual habits, political duplicity, such as in the Urbane:
All the Gauls did Caesar vanquish, Nicomedes vanquished him;
Lo! now Caesar rides in triumph, victor over all the Gauls,
Nicomedes does not triumph, who subdued the conqueror."
Men of Rome, keep close to your consorts, here's a bald adulterer.
Gold in Gaul you spent in dalliance, which you borrowed here in Rome."
I've been looking into whether this was common and considered a perk of the legions throughout the Roman Republic and Empire.
Do we have any records of this going back to, say, Marius?
Was this still permitted during a triumph of a later princeps? What about during the dominate? Are there records from that time (even if disapproving, such as by Christian writers)?
An interesting article here: Roman Military Research Society, especially on the scarcity of sources and attempts at reconstruction.