In my personal experience these newer "talking alarms" normally involve a flashing light and an alert and in a large establishment where everyone needs to be aware but not immediately evacuated they can be beneficial but for the circumstances you have described (and personal family circumstance) anything that is "not normal" triggers a response from those on the autistic spectrum.

I spend my life on egg shells ensuring situations that may provoke a response are communicated well in advance of the actual event so that when it comes time there has been adequate opportunity to adjust -  unfortunately an alarm has no chance of being introduced and is either there or not.

Regardless of what alarm system is chosen it is likely your arrangements will require a building sweep probably by a team rather than an individual

As someone with 40+ years experience in fire safety and 29 years of having a son on the autisic continuum, I strongly suggest you contact professionals in this field before taking any steps down the voice alarm route.

People with autism do not fit into one single pigeon hole when it comes to listing their obsessions or phobias. Indeed, some may not demonstrate any phobic behaviour 

When he was young, my son was absolutely freaked out by unexpected sounds of a certain pitch. Buzzing bees and hand dryers were his enemies. But I have met people on the spectrum who have had reactions to various stimuli - including various noises. It is entirely possible that even a recorded voice alarm may trigger some into freezing, panicking and action contrary to a safe and rapid evacuation.

If the premises is in place to care for severely autistic persons, this could be very problematic.

My own view is that I would consider playing music - a particular tune that staff and others would recognise as the fire alarm. I can see no reason why the music couldn't start fairly quiet and build to the required sound pressure levels (65 dBA - or 75 dBA for sleeping risks) over say 10/15seconds so people are not shocked by a sudden sound noise. There could be an element of a recorded message, perhaps recored by a familiar member of staff to help reduce anxiety.

I recall a nightclub with around 1200 drunk party-goers having a system where the DJ would play the Dr Who theme in an evacuation. It was to prevent a panic/crush situation as it the theme tune informed staff that an evacuation was about to happen and they needed to take up certain positions. If it can be done to avoid panic in a club, why not for those with special needs????

This system would need to be recorded as a variation from the British Standard (5839-1). It may be useful to talk to the local fire service in terms of it being acceptable to them enforcement wise. I cannot see it being an issue

My advice would be to contact competent professionals - for example at the National Autistic Society http://www.autism.org.uk for advice. The NAS run homes for those with autism and will have experience with fire alarms.

I might also contact a voice alarm manufacture such as Baldwin Boxall . I recently worked on a major project with them which required a number of bespoke solutions. I am sure they would be able to guide you. http://www.baldwinboxall.co.uk