A key-tone preference may have been changed or muted. Sound and successful control selection are separate observations.
What the observation means
Some hobs keep warning sounds even when ordinary key tones are disabled. A reset or another user may alter the preference.
A useful next step
Check the normal settings menu and watch the display to confirm selections. If important alarms are also absent or controls become unreliable, request advice. Do not assume silence means the hob is off, and do not rely solely on sound when checking which zones remain active.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.