I remember wanting to play Soul Reaver badly but I couldn’t even get past the very beginning because all the instructions were voiced during gameplay.
needswhippedcream, via Reddit
Essential information means something you absolutely can’t play the game without.
Conveying essential information by sound alone is an obvious barrier for people with physical hearing impairment, but is it also causes problems for situational impairments, such as playing in a noisy environment, through poor quality speakers, or with sound muted to avoid waking the baby.
The obvious way in which this is achieved is subtitles, but many sounds other than speech are also essential to gameplay. Such as for example a siren alerting a player an imminent event.
An easy test is to ask someone to play through for the first time with the sound muted. If at any point they are unable to progress due to information being missed, it needs to be conveyed by another means, such as an icon, visual prompt, or text prompt.