Sound Is Never the Only Channel
MUST: Give every audio cue a visual equivalent — sound is a reinforcement channel and never carries feedback alone (WCAG 1.3.3). A Submit whose only success signal is a chime does nothing observable on a muted tab, and nothing at all for a deaf user. Removing the sound must cost nothing.
Every audio cue needs a visual equivalent — sound supplements feedback, it never carries it
Bad
Good
Why it matters
Cherry-picked from Raphael Salaja's sounds-on-the-web skill (rule `a11y-visual-equivalent`). The failure is easy to reproduce and easy to miss in development: a Submit button whose only success signal is a chime. On a muted tab — which is the default state of most tabs most of the time — pressing Submit produces no observable change at all, and the user presses it again.
Deaf and hard-of-hearing users get the same nothing, permanently. This is WCAG 1.3.3 Sensory Characteristics: instructions and feedback must not depend on a single sensory characteristic. It is the same principle as "Redundant Status Cues", which is scoped to colour and WCAG 1.4.1 — one channel, one criterion, identical shape. Sound is simply the channel that is switched off far more often than colour is.
The rule is not "no sound"; sound is a genuinely good second channel when the user is not looking at the screen. The rule is that removing it must cost nothing.