It is also strange to think of there being more instances of characters talking in this film, and I suspect that would have diminished the impact of vocal speech.
When watching A Quiet Place, you find your body rigid and your breathing quiet, every sound causing you to hold your breath.
So when the characters do speak, it is meaningful and jarring, the human voice sounding almost strange as it breaks the silence.
Too much talking, even in flashback, would have robbed this film of some of its potency.