The epub-renderer in our Sony's use it's own font, if no font is embedded into the epub.
This standard-font dosn't cover most of the special characters.
Try Settings / PRS+ settings / Book Viewer Settings / Use EPUB Style (CSS File) and select FontsAsInLRF.css
to use PRS+ enhanced fonts also with epubs.
Please note that you will have to open another epub and then go back to your original, as epub-user-styles are only applied when a book is opened.
If also the enhanced fonts supplied with PRS+ will not cover the needed glyphs, you will have to locate the font-file (*.ttf) on your computer.
Copy this file to a folder /font/ in the root of IM (Internal Memory)
and create your own custom *.css file.
This file had to be placed in /database/system/PRSPlus/epub
see this thread
Override ePub CSS with userStyle.css? for the needed format, especialy post #11 by kartu
The path to a font-file in a folder /font/ in the root of IM will read as:
src:url(res:///Data/font/YOURFONTHERE.ttf);
HTH
Mark