Your DECtalk Express has old firmware. Use the Emacspeak
command `C-e d V' to find out your version. You should be running a
version no older than 4.2bw from March 1995. If you have an earlier
version, you can find an updated version at
http://www.ultranet.com/~rongemma/tips_upd.htm, a WWW
site maintained by Ron Jemma of the Dectalk Group at DEC.
Alternatively, you can send email to Anne Nelson at
firstname.lastname@example.org. The most recent version at this
writing is 4.3 release AA X01 May 20 1996.
The problem is due to remaining bugs in the Dectalk firmware. When emacspeak produces tones, especially when split caps is on, the dtk sometime goes into squealing mode.
If you notice this happening in particular text documents, just turn off split caps mode locally with `C-e d s'.
It's completely insignificant. autorevert is a new minor mode provided by emacs 20 and emacspeak looks for it and if found speech-enables it.