The way Slavonic is pronounced in various countries in indeed different. I have learnt the Serbian pronunciation and my friends in Bulgaria make jokes about that (BlagOslovjen jEsi GOspodi...
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Serbian language has a system of rising (acute) and falling (gravis) accents combined with two lengths of vowels, that together make four accents - short rising, long rising, short falling and long falling. Further more, long vowels don't necessarily have to be accented which makes Serbian language & pronunciation even more complex.
The majority of other indoeuropean languages have only falling accents. Two rising accents in Serbian usually come one syllable before the "old" falling accent. Therefore, whatever you do with Serbian pronunciation while chanting, someone's ear will not be pleased.
Sadly the Serbs made the use of Mokranjac' s music compulsory and due to the country's ecclessiastical isolation studies in psaltic music have rather stagnated, though there are a few notable exceptions...
I think what Markellos has said about the complexity of practical use of the chant in Russia, could have been said for Serbia as well.
Throughout the history, ecclesiastical music in Serbia was not considered to be any different from "Byzantine" in general. Up until the end of 18th century, if they have used notation at all, Serbian chanters have been chanting from neumatic notation. The very last school of Byzantine music (old notation) in Serbian church has been established in 1819 in Budapest (there was a significant Serbian community at the time).
Serbian music begins to develop independently since the beginning of 19th century. The period since 1804 to 1838 was the time of continuos struggle against the Turks, and it happened at the same time with the adoption of the new method. Serbian chanters, alas, have never accepted Chrysathos notation, but have rather forgotten the old one instead.
Finally, Serbian chant was installed in seminaries as a local version of "the Greek chant" as it used to be called. It was thought that any unified system is better than no system at all. Therefore, Serbian chant was introduced to the churches in what today is known as FYROM.
Mokranjac has written down what has been chanted in churches almost a century later. The similar situation was in Austro-Hungarian parts of Romania, where Cuntana has done exactly the same job. Sometimes Mokranjac & Cuntana melodies differ only in language.
However, I don't recall any Serbian "ecclesiastical isolation" at any time. What did you have in mind actualy?
Can you please say who are "a few notable exceptions"? Did you mean historically or contemporary?
Thank you