I suspect that the US, including the upper class that can still afford jet air travel, contains more cultural diversity than most other countries, and what you're noticing is the outliers on the right side of the noisiness distribution.
Me, I find Buenos Aires to be particularly loud, but it's only occasionally people (usually groups of cacerolaceros, other piqueteros, or football fans); mostly it's the huge superchargers on the buses, the garbage trucks, the pneumatic actuators on the buses without silencers installed, the motorcycles without mufflers installed, the cumbia villera from the cellphones, and so on. In fact, the only louder city I've ever traveled to was Lima.