The FAA gives one requirement for the autopilot in the following FAR...
FAR §25.1329 Flight guidance system.
(j) Following disengagement of the autopilot, a warning (visual and auditory) must be provided to each pilot and be timely and distinct from all other cockpit warnings.
Why is an auditory alarm necessary when the autopilot disengages?
Was there a particular accident where the pilots ignored visual warnings?