Although I consider myself the current maintainer of emacspeak, I don't
know the answer to the question yet, but I believe it all depends on
which program or device you use for the speaking. Please at least have a
look at your /etc/emacspeak.conf file and it shows you that you can
experiment with environment setting before you really implement it in
/etc/emacspeak.conf. I think if you are using a software tts you should
give the right settings for that tts in either env or in
/etc/emacspeak.conf.
Please follow up here if you found out or if the above information was
not enough.