Early accounts of reactions to gunpowder in Mesoamerica are... not particularly notable, to say the least. They occur in the midst of battles and, while the harquebus volleys were effective, they were not bewildering. Take this account from Bernal Diaz del Castillo of his time with the 1517 Cordoba Expedition:

After the flight of arrows, the warriors, with their feathered crests waving, attacked us hand to hand, and hurling their lances with all their might, they did us much damage. However, thank God, we soon put them to flight when they felt the sharp edge of our swords, and the effect of our guns and crossbows, and fifteen of them fell dead.

That passage -- aside from being very indicative of the mix of dramatization and peculiar precision of DdC -- does not exactly fit into the idea of Americans being cowed or stunned by guns. We can see this even more clearly when DdC describes an early battle of the Cortes Expedition.

We our muskets and crossbows and with good swordplay we did not fail as stout fighters, and when they came to feel the edge of our swords little by little they fell back, but it was only so as to shoot at us [with arrows and slings] in greater safety. Mesa, our artilleryman, killed many of them with his cannon, for they were formed in great squadrons and they did not open out so that could fire at them as he pleased, but with all the hurts and wounds which we gave them, we could not drive them off.

So there we have an account of guns and even a cannon being used, but resulting only in strategic retreats and continued attack. DdC even describes the Americans disguising the effect of the guns and artillery:

when we fired shots the Indians gave great shouts and whistles and threw dust and rubbish into the air so that we should not see the damage done to them, and they sounded their trumpets and drums and shouted and whistled and cried "Alala! Alala!"

If you want to get a more visceral reaction to gunpowder, the demonstration held by Cortes for the ambassadors of Motecuhzoma is one of the better documented and dramatic. Several calpixque of the Triple Alliance arrived in Cempohuallan, where Cortes had allied with the local ruler, on a scouting mission. While there, Cortes arranged for firing of heavy cannon and a display of horsemanship.

The most dramatic account comes from Sahagun:

Especially did it cause [the calpixque] to faint away when he heard how the gun, at [the Spaniards'] command, discharged [the shot]; how it resounded as if it thundered when it went off. It indeed bereft one of strength; it shut off one's ears. And when it discharged, something like a round pebble came forth from within. Fire went showering forth; sparks went blazing forth. And its smoke smelled very foul; it had a fetid odor which verily wounded the head. And when [the shot] struck a mountain, it was as if it were destroyed, dissolved. And a tree was pulverized; it was as if it vanished; it was as if someone blew it away.

Now, Sahagun's Historia general... while written by Nahua scribes in Nahuatl (though under the supervision of Spanish priests), was not written by anyone with first hand account of this event. While the event is corroborated with other accounts. it's also very heavily dramatized. Particularly the part about the calpixque "fainting" is a more than a little suspect.

Gomara's biography of Cortes, for instance, simply has the ambassadors diving to the ground upon the firing of the cannon, an understandable reaction. DdC does not even mention this, though he does remark that firing off the cannon gave the Americans a fright. He also take a moment to mention that the cannons had been loaded with "a great charge of powder, so that they should make a great noise when they were fired off."

They idea that Americans generally went about awed and stunned by the gunpowder is one of those orientalist narratives that pervade this era of history. While guns, and particularly cannons, could have an intimidating effect, we see rapid adaptation and adoption by American groups. While DdC was able to remark upon the devastating effect artillery had on the massed troops at the start of Cortes' campaign, by the time the Spanish-Tlaxcalan troops were actually attacking Tenochtitlan, the Mexica troops were approaching in zig-zags, hitting the deck, and opening ranks to stymie the effects of cannon. Particularly among groups in Mesoamerica accustomed to barrages of arrows and sling bullets, accompanied by blaring horns and whistles, with smoke from signalling wafting across the battlefield, the introduction of guns were not so alien as to be overpoweringly strange.