An Australian blind want to hear Australian English,
even if the letter is from US.
The common error with blindness and such is people tend to think
that blind and non-blind are a lot different. They are not.
if the original message content was digitized voice, then a
mapping to australian accent will be an interesting technical
which has nothing to do with blindness.
If the original message content was text, the software might
well need to know that US english spellings and idiom are
involved so that it can use a relevant set of "dictionarys".
Just as an Australian non-blind can read en-us mails without
content language information, an Australian blind can recognize
it without content language information.
Still, that will be an issue for the presentation software,
presumably the user will have the ability to choose a voice
of high/low pitch, and, depending on sophistication of the
technology and the particular software, possibly also a
choice of accents.
It is a poor quality presentation system if it needs such a configuration.