@Toyu_Frey By the way, here's another relative clause from your linked page:

（僕が話しかけた）瞬間、～～ = The moment (when I talked to her), ~~

僕が話しかけた modifies the noun 瞬間.

And one more...

（少女の持つ）ナイフ = the knife (which the girl has)

The relative clause 少女の持つ modifies the noun ナイフ.

I see a lot of relative clauses used in the text on that page..

This whole line is a noun phrase: 妙になれなれしい, ふざけたような口調. But it's more natural to translate it as a normal sentence (Subject+Verb+...) when translating into English, so you translated it as "His voice was...", am I right?

(rather than translating it literally as a noun phrase, as in "A strangely over-familiar way of speaking, as if joking.")