For further research on voice alarm systems you could try the BFPSA Code of Practice for the Design, Installation & Servicing of Voice Alarm Systems.
or Psychological Aspects of Informative Fire Warning Systems published by the BRE,
or Human Behaviour in Fires a Summary Report by Jonathan Simes,
or BS5839 Part 8. Code of practice for the design, installation and servicing of voice alarm
systems.
There was a considerable amount of research carried out some years ago and much of the reported research was very interesting and appeared to strongly favour its use, although it does not seem to have been taken up by occupiers or installers as expected. I have seen it in operation recently within a school and it was reported to be very effect by all participants, sadly there seems to be a cost implication restricting its installation and it was subsequently replaced by electronic sounders.