There are two kinds of passive voices in German, one uses a form of "sein" as an auxiliary verb, and the other one uses "werden".
As a rule of thumb, the form with "sein" has a perfect aspect to it, whereas the form with "werden" does not. I think a comparison with English gives you a good idea:
[EN - active voice] I am reading the book/I read the book.
[EN - passive voice] The book is being read/The book is read.
[DE - active voice] Ich lese das Buch.
[DE - passive voice] Das Buch wird gelesen.
Unlike English, there is not distinction between ongoing and habitual actions in German grammar. You need to use auxiliary verbs (or dialect forms).
A bit curiously, there is a passive voice with perfect aspect. You don't really have that with active voices in German, so this is a bit odd. Nevertheless, it is commonly used and understood.
[EN - passive voice] The book has been read.
[DE - passive voice] Das Buch ist gelesen.
My gut feeling is that the German passive voice is used when you talk about results that have already been achieved and which are still in the making, so German has perfect/impefect aspect here.
In your example, the English translation is as follows:
Wie wird "eu" in Feuer ausgesprochen? - How is "eu" in Feuer pronounced?
*Wie ist "eu" in Feuer ausgesprochen? - How has "eu" in Feuer been pronounced?
In fact, the last sentence sounds borderline ungrammatical to me.