I've heard of a captain, in his haste, initiating an emergency descent mid Pacific Ocean following an unrelated cockpit aural warning
That was an interesting incident having seen the report. It was not strictly true to say it was an unrelated cockpit aural warning however. The aircraft departed at midnight for an island destination four hours away. Same aircraft had a pressurisation defect on earlier flight that day and written up on last page of maintenance document. New maintenance document ready for the crew and no reference of earlier problem. In other words the crew were unaware of the earlier defect. Climbing through 29,000 ft the cabin altitude warning horn sounded. No serious pressure changes noticed in ears. Cabin altitude noted going though 12,000 but not rapidly. All previous simulator training in this company included only rapid depressurisation (not uncommanded pressurisation change which is a more leisurely event) and woe betide any crew if any delay in emergency descent.
Accordingly, the captain directed the F/O to carry out the memory items which included closing the outflow valve (manual operation) while captain started down with no delay.
Severe ear distress experienced and a closer look at cabin altitude a few seconds after descent had got going, showed 3000 ft cabin altitude which meant cabin altitude had gone from 12,000 ft to 3000 ft in around two minutes - explaining ear distress. Aircraft levelled around 23,000 ft and situation evaluated. The full closing of the outflow valve had caused the cabin pressure to increase causing ear distress. All this being wise after the event. .
Aircraft did 180 and returned to base landing 20 minutes later. . Investigation revealed the replacement cabin controller from the earlier flight which was a spare from the flyaway kit, was filled with moisture from being in a plastic bag in the aircraft hold. Another cabin controller was installed and the flight proceeded normally. The emergency descent was never needed because the symptom was an uncommanded pressurisation change (albeit a gradual one) until the cabin passed 10,000 ft which triggered the warning horn. The captain accepted full responsibility for the incorrect decision to treat the event as worthy of a rapid descent when it should have been treated as only an uncommanded change of cabin altitude and that particular non-normal checklist applied. Paucity of proper training in the simulator for this type of non-normal set the scene for a stuff up and was no help.