Is there some guide for pronouncation? Since I pronounce Reimu the same way...
I however pronounce Cirno as 'Cher-no', Sanae as 'Sa-na-e~', Eirin as 'Eye-rin', Mokou as 'Mo-ko~' and Keine as 'Kein'. Also, I pronounce Byakuren as 'Bi-a-ku-ren'. Is that correct? Not sure on Koishi, Reisen and Youmu either. I pronounce those as 'Koy-shi', 'Righ-sen' (as in 'right' and 'high') and 'You-moo'.
Japanese is pronounced exactly same way every time.
あ - A (pronounced AH, as in "father")
い - I (pronounced EE, as in "leer")
う - U (pronounced OO, as in "cool")
え - E (pronounced EH, as in "kettle")
お - O (pronounced OH, as in "bore")
Every vowel is it's own syllable. So, for example, "Keine" would be "Keh-ee-neh."
"Eirin" would be "Eh-ee-rin."
"Reisen" would be "Reh-ee-sen"
"Youmu" would be "Yoh-oo-mu"
"Byakuren" would be "Byah-koo-ren" (note that "bya" is one syllable)
For more information on Japanese syllabary, read
this.