moslem seems to be an older spelling common in the last century and before, but i can't recall hearing it spoken now (and knowing they meant to say "moslem"), so no word as to how it's pronounced. for the most part i encountered it in older books.
my first instinct is to guess the first vowel "O" is not a schwa as in "muslim", but a more pure [o] sound as in "low", the rest of the word being pronounced the same.
there are many ways to transliterate arabic, and this change may simply be a reflection of the ways those methods have evolved. some dialects of arabic do not differentiate between [o] and [u], unlike modern hebrew that does.
being a semitic language, vowels are not emphasized in writing.
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הייתי צוללת עכשיו למים
הכי, הכי עמוקים
לא לשמוע כלום
לא לדעת כלום
וזה הכל אהובי, זה הכל.