Of the classical linguistic categories, English does not have paradigmatic representations of Voice, Mood, or Aspect for verbs, nor Case or Gender for nouns. It has two paradigmatic tenses, present and past, and many types of tense-related constructions. But that's it.
So don't worry about English Voice. There isn't any, just like there isn't an English Perfect Tense or an Inceptive Aspect or a Benedictive Mood. In English the Passive is a Construction, not a mood or a voice or a tense, and it defines a "passive clause" as one containing a Passive Construction.
Similar remarks can be made about the Progressive Construction and the Perfect Construction. They all require specific auxiliary verbs, they all require specific verb forms after the auxiliary verbs, they each occur in a specific order in the verb phrase, and the Passive -- only -- changes the noun phrase arguments, promoting the old object to subject, and demoting the old agent subject to an optional by-phrase.
German is much the same as English in this regard, but has fewer irregularities (this is made up for by noun genders and noun plurals). Werden is used -- "become" instead of just "be" -- but it's otherwise the same. Though, once again, German morphology comes in handy; the ge- prefix marks participles unmistakeably, and the -t/-en suffix distinguishes regular from irregular verbs. That's Germanic.
But there are other language families. The Austronesian family is famous for having its own ways of handling passive; Malagasy has four different types of what can be called "Passive" constructions. Indonesian has several, and so does Acehnese. And even in English, there are constructions that are similar to Passive, like Her book is selling well, which is an example of the Middle Alternation, a different thing in English.
All of this doesn't go back far enough, though, since as Geoff Pullum points out, far too many people think that "passive" is an emotional term, not a grammatical one. Consequently, like Strunk and White, they can't tell a passive from a hole in the ground.