Nothing, but the SAPI voice engine that is used its very simple.
You only call a function speak("Blabla") and then it comes with the current settings out of your headphones. So if there is a voice for sapi that can spell Cyrillic symbols this will work, if not then you can't spell this with sapi.
This plugin is only a bridge between miranda and the microsoft api for text to speak.
In German the letters äöü make many trouble as well, so if the voice is for english people it will not understand that symbol and you hear funny parts or nothing. Badly i don't know much about the sapi voices itself, so i don't know if there is a solution for your problem.
i saw a plugin that converts Cyrillic symbols into text, maybe we can combine both. So if the other plugin provide an service for converting text, and this plugin detects that the other plugin is present, then all text will be converted before it will gave to sapi.