Of the 29 Proto-Semitic consonants, only one has been lost: */ʒ/, which merged with /ʃ/. But the consonant */ʒ/ is still found in many colloquial Arabic dialects. Various other consonants have changed their sound too, but have remained distinct. An original */p/ lenited to /f/, and */ɡ/ – consistently attested in pre-Islamic Greek transcription of Arabic languages – became palatalized to /ɡʲ/ or /ɟ/ by the time of the Quran and /d͡ʒ/, /ɡ/, /ʒ/ or /ɟ/ after early Muslim conquests and in MSA (see Arabic phonology#Local variationsfor more detail). An original voiceless alveolar lateral fricative */ɬ/ became /ʃ/. Its emphatic counterpart /ɬˠ~ɮˤ/ was considered by Arabs to be the most unusual sound in Arabic (Hence the Classical Arabic’s appellation لُغَةُ ٱلضَّادِ lughat al-ḍād or “language of the ḍād“); for most modern dialects, it has become an emphatic stop /dˤ/ with loss of the laterality or with complete loss of any pharyngealization or velarization, /d/. (The classical ḍād pronunciation of pharyngealization /ɮˤ/ still occurs in the Mehri languageand the similar sound without velarization, /ɮ/, exists in other Modern South Arabian languages.)
Other changes may also have happened. Classical Arabic pronunciation is not thoroughly recorded and different reconstructions of the sound system of Proto-Semitic propose different phonetic values. One example is the emphatic consonants, which are pharyngealized in modern pronunciations but may have been velarized in the eighth century and glottalized in Proto-Semitic.
Reduction of /j/ and /w/ between vowels occurs in a number of circumstances and is responsible for much of the complexity of third-weak (“defective”) verbs. Early Akkadian transcriptions of Arabic names shows that this reduction had not yet occurred as of the early part of the 1st millennium BC.
The Classical Arabic language as recorded was a poetic koine that reflected a consciously archaizing dialect, chosen based on the tribes of the western part of the Arabian Peninsula, who spoke the most conservative variants of Arabic. Even at the time of Muhammed and before, other dialects existed with many more changes, including the loss of most glottal stops, the loss of case endings, the reduction of the diphthongs /aj/ and /aw/ into monophthongs /eː, oː/, etc. Most of these changes are present in most or all modern varieties of Arabic.
An interesting feature of the writing system of the Quran (and hence of Classical Arabic) is that it contains certain features of Muhammad’s native dialect of Mecca, corrected through diacritics into the forms of standard Classical Arabic. Among these features visible under the corrections are the loss of the glottal stop and a differing development of the reduction of certain final sequences containing /j/: Evidently, final /-awa/ became /aː/ as in the Classical language, but final /-aja/ became a different sound, possibly /eː/(rather than again /aː/ in the Classical language). This is the apparent source of the alif maqṣūrah ‘restricted alif’ where a final /-aja/ is reconstructed: a letter that would normally indicate /j/ or some similar high-vowel sound, but is taken in this context to be a logical variant of alif and represent the sound /aː/.
Although Classical Arabic was a unitary language and is now used in Quran, its pronunciation varies somewhat from country to country and from region to region within a country. It is influenced by colloquial dialects.