kitto odoroki-sugite koe mo denakatta no ne! きっと驚き過ぎて声も出なかったのね！
[I'm sure that] [he was] so surprised that [he couldn't even say anything], [wasn't it?]!
odoroki-sugiru 驚き過ぎる
To be very surprised. To be extremely surprised. (compound verb.)
koe ga denai 声が出ない
Voice can't leave [his mouth].
[His] voice doesn't come out.
[He] can't say anything.
[He] was speechless.
janai to, ano han'nou wa setsumei ga tsukanai mono! じゃないと あの反応は説明がつかないもの！
Otherwise, that reaction can't be explained!
____ wa setsumei ga tsuku 〇〇は説明が付く
The explanation of [something] is settled. (double subject construction.)
[Something] is explained.
mono もの
Thing. (noun.)
As a sentence ending particle, denotes the current sentence is a rationale. In this case, "the reaction can't be explained" is the reason why Teruhashi believes "he was so surprised he couldn't even say anything."
