And the CVR did record a chime: it was there, right after the AP disconnect cavalry charge. And there were further chimes throughout the recording. None of the pilots mentioned any EICAS or PFD warnings though. They remained focused (mesmerized) trying to correct aircraft attitude.
NO, thats not the chime. The chime to look for would be right before the Captain said "what?"
The EFIS Comparator constantly monitors both sides and looks for disparities between the two. The chime would be the very first indication of an anomaly in the AHRS/IRU. The caution chime is followed by a flashing yellow push-to-cancel caution light on the glareshield and a red HDG/ATT flag on the primary flight display, with an associated EFIS COMP MON and possibly AHRS/IRU1 or 2 displayed on the EICAS.
So, maybe time to add a third system and use it as a tie-breaker when the two main systems disagree
It's already there, called the standby attitude indicator. The standby is a totally independent third system right in the middle of the cockpit. Some are a very nice digital ISI, others on the older A/C would be the steam gauge type.