Why is an auditory alarm necessary when the autopilot disengages? 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?
Yes, there have been several accidents caused when the autopilot has been inadvertently or automatically disconnected and the flight crew have failed to realise it happened. Even with the audible alert, it's reasonably common, but experiments in controlled conditions demonstrate that alerts provided in only one medium (just visual, or just auditory) are not reliably noticed in times of high workload.

Here's an interesting accident report I remember reading at the time, where the flight crew apparently failed to notice that they had deliberately disconnected the autopilot. For more on inattentional deafness, see the paper Failure to Detect Critical Auditory Alerts in the Cockpit: Evidence for Inattentional Deafness.