With many airlines it seems to be standard that the Pilot Monitoring observes the Flight Mode Annunciator (FMA) and calls out any changes in the autopilot mode displayed on the FMA (I guess to raise awareness to the new or armed flight mode). Why are these announcements not automated (i.e. computer generated voice like the altitude announcements during final approach)?
The responsibility of checking the FMA after a mode-change is not the PM's alone. If the PF calls for VNAV, the PM will select VNAV, now both of them need to check VNAV is active.
Regarding aural alerts, there are already alerts to when the flight deviates from the norm, for example an altitude warning (which doubles as approaching target altitude), stick shaker, low-speed / over-speed, etc.
But, your question is about changing the flight-mode, say from VNAV PTH to VNAV SPD, or VNAV to FLCH. As mode changes are a direct consequence of crew commands, I don't see a benefit.
One downside I can think of is that the ATC will hear a lot more aural warnings as the crew changes the modes while reading back an instruction.
As for the effectiveness of such system, say in case the crew pressed the wrong button and they were too tired to double-check the FMA, they shouldn't be flying in the first place.
The analysis of accidents and numerous scientific publications have demonstrated that operational stress and fatigue can result in pilots neglecting crucial information, such as visual and aural warnings. This phenomenon, called channelized attention is defined as being “the allocation of attention to a particular channel of information, diagnostic hypothesis or task goal, for a duration that is longer than optimal, given the expected cost of neglecting events on other channels, failing to consider other hypotheses, or failing to perform other tasks” (Wickens, 2005). It seems possible that this phenomenon might occur during the go-around phase which is often unexpected, uncertain and of sudden onset, and may take place after many hours of flight.
That's from a BEA study on Aeroplane State Awareness During Go-Around.
Too many alerts and any human will become used to the regular sound with mode changes regardless of what the aural warning says. Eventually it'll become background noise.
The ideal level of warnings is not to overload all the senses, and let the aural warning be when something is crucial, so the training can kick in with speedy recovery actions versus a relatively slow diagnosis of the situation.