Friend Oli's points about trumpeters are well–taken.
The French and Austrians engaged in numerous exchanges just before Wagram and during the retreat/pursuit thereafter. Although details of the actual tactical mechanics are sparse, trumpeters are specifically mentioned in several cases. Massena, for instance, charged with returning captive FML von Weissenwolff to the Austrian side, had to demur at one point because the combat was too intense 'to risk a trumpeter.'
On 11 July, when the battlefield ceasefire was declared at Znaim, Jean Baptiste Fournier of Arrighi’s staff remembered ‘the aides–de–camp or officers of the emperor’s staff were seen riding rapidly, with a white handkerchief tied to the arm...' (Souvenirs, Maurepas: La Vouivre, 2009, p. 32) This, however, seems to have been to deliver the news within the French lines, not to meet with the Austrians.
However, being one of these emissaries could be a dangerous business as Hauptmann d'Aspre on the Austrian side and Capt. Marbot from Massena's staff discovered when both were wounded on 11 July trying to get the troops to cease firing on the highway south of Znaim.