Doubts, page-36

Only thing I could find on coughs/ accents from a linguist

A cough does not carry an accent. However, the vocal tract would amplify and dampen certain frequencies of noise during a cough that are in line with what you would hear in speech just as if that person were phonating. This is due to the shape of the resonating chamber that is the vocal tract that is individual to each person. So if you know the person, you might be identifying that person based on the resonant characteristics of that person's vocal tract that you may be familiar with through speech.
Another thing that may account for this is that there may be different cultural norms about coughing that you may be picking up on (such as different ways of covering your mouth or holding in a cough that may be different in different cultures). I have no idea if there are actual cultural differences here, but it is one option that comes to mind.

Bear in mind that variation within a population (individual differences) usually greater than variation between populations and given how diverse the US pop is almost a given