An alternative approach with non-biological causes: The generation ship was prone to catastrophic loss of air pressure. Automated systems and rapid response from the staff meant pressure loss didn't last more than a few seconds each time (and people can survive that, it's just uncomfortable), but the air pressure loss was significant when it occurred. Thanks lowest bidder contracting!

Most of the negative effects of brief exposure to near-vacuum conditions are unpleasant, but not long term. But unfortunately, your sinuses are bad at venting pressure quickly; when the pressure drops rapidly from 14.7 psi to less than 5 psi, your eardrum is almost guaranteed to rupture. Now, any given rupture generally heals within a few months, but unfortunately, the steady drip of air pressure problems means they usually got reruptured over and over before the healing completed, and eventually the scar tissue rendered stiffened the eardrums to the point of deafness.

So after a few decades of this, your skeleton crew arrives at a new planet stone deaf, having long ago discarded spoken language in favor of text and sign communication as they all went permanently deaf.

As it happens, all the embryos have perfectly functional ears. Unfortunately, no one thought to send along recordings of the language, and everyone who knew how to speak it is decades out of practice, and can't actually hear what they're saying; the newly decanted babies simply don't have any consistent model for speaking the language. So they learn sign instead. And it works. And reinventing spoken language from scratch is not that easy, and when it happens, each local group does it independently, ending up with mutually unintelligible spoken language, so sign becomes the lingua franca. Even if the occasional group speaks aloud amongst themselves, they have little choice but to fall back on signing (which everyone inherited from the original skeleton crew) when dealing with outsiders.

Add on some additional motivations for signing (e.g. the planet itself has a low pressure atmosphere or constant high winds which reduce how well sound carries and/or require respirators that muffle your voice, or have dangerous critters that attack noisy folks), and the system can end up self-sustaining for quite a while.

An alternative approach with non-biological causes: The generation ship was prone to catastrophic loss of air pressure. Automated systems and rapid response from the staff meant pressure loss didn't last more than a few seconds each time (and people can survive that, it's just uncomfortable), but the air pressure loss was significant when it occurred. Thanks lowest bidder contracting!

Most of the negative effects of brief exposure to near-vacuum conditions are unpleasant, but not long term. But unfortunately, your sinuses are bad at venting pressure quickly; when the pressure drops rapidly from 14.7 psi to less than 5 psi, your eardrum is almost guaranteed to rupture. Now, any given rupture generally heals within a few months, but unfortunately, the steady drip of air pressure problems means they usually got reruptured over and over before the healing completed, and eventually the scar tissue stiffened the eardrums to the point of deafness.

So after a few decades of this, your skeleton crew arrives at a new planet stone deaf, having long ago discarded spoken language in favor of text and sign communication as they all went permanently deaf.

As it happens, all the embryos have perfectly functional ears. Unfortunately, no one thought to send along recordings of the language, and everyone who knew how to speak it is decades out of practice (or dead, if the original crew died en route to be replaced by children born in space), and can't actually hear what they're saying; the newly decanted babies simply don't have any consistent model for speaking the language. So they learn sign instead. And it works. And reinventing spoken language from scratch is not that easy, and when it happens, each local group does it independently, ending up with mutually unintelligible spoken language, so sign becomes the lingua franca. Even if the occasional group speaks aloud amongst themselves, they have little choice but to fall back on signing (which everyone inherited from the original skeleton crew) when dealing with outsiders.

Add on some additional motivations for signing (e.g. the planet itself has a low pressure atmosphere or constant high winds which reduce how well sound carries and/or require respirators that muffle your voice, or have dangerous critters that attack noisy folks), and the system can end up self-sustaining for quite a while.