I’m tone-deaf. You give me a sound puzzle without any visual component and I cannot do it.
drownedout, via forums.unknownworlds.com

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 loss, 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 sounds other than speech can also be essential to gameplay. For example a siren alerting a player an imminent event, or an audio based puzzle.

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. Usually visually, through an icon, effect, text, or other visual device.

Best practice example: Fortnite