I am afraid this question is going to be rather specific and perhaps off-topic, but I hope some of the Italian users (at least) can help me out with this curiosity.
I have always been used to the Italian pronunciation "de ics" of the differential $dx$. In principle there would be nothing wrong in saying "di ics", as the letter "d" is pronunced "di", but then why do the Italians say "de ics"?
It largely depends on where the $dx$ is. As a differential, it is often pronunciated [di: iks] or [de iks]. When it's used in the derivative $\frac d{dx} $ it's almost always [de iks], since the whole thing is pronunciated [de in de iks].
$x$ is never [ikse] unless one wants to mock some regional dialect.