Wagons are sturdy, 4 wheeled vehicles that usually use two-three oxen or some other animal to pull. But--what sounds do they create when they pull? If they are tired, would they grunt? I mean not grunt but something like it? I just can't think of some words to describe the animals themselves and their troubles.
The irregular thud of heavy road apples flattening on the ground for one. It depends largely on what kind of surface they're on. Also, I have no idea if oxen were shod or not, I mean with iron or steel shoes, like horses. I suspect not. But on cobblestone the hooves would make a much harder clomp or clack, a sharp clatter if they do have metal shoes (which I doubt). On a dirt road just some thumps like a muffled drum being struck softly. Thick heavy breathing, getting louder as they get increasingly worn out, and punctuated by sudden loud gusts through open mouths, maybe now and then with flapping lips. Hissing through nostrils or chuffing through open mouths, maybe steam gusting forth if it's cold enough. Yeah, maybe a grunt or groan every now and then when things get rough, like up a hill or something. The buzzing of flies, maybe the louder buzzing of wasps or bees now and then. And just for visual atmosphere don't forget the occasional twitching of thick hide, especially when getting stung. Oxen don't have tails they can swat at pest with, do they? I think they just have short stumps? And of course the overpowering smells. Refer back to road apple statement for the strongest one. But that heavy, musky animal smell and a smell of maybe something like stale hay or whatever they eat. You'd need to research that one, I don't know. Then of course a symphony of creaks, squeaks and groans from the tortured wood of the cart, the yoke, and the leather or rope harness—especially when navigating the potholes or turns. The harder clatter of the wooden wheels (unless it's a dirt road, then just occasional thumps as they hit ruts and holes) or much louder clashing of steel or bronze bound wheels if that's the kind they are. And the short whish-thwack! whenever the driver uses his crop or sharp crack of the whip or whatever he goads them with.
