While the adhan's text is essentially fixed across the Muslim world, with only minor variation between Sunni and Shia practice, the melodic style in which it is delivered varies considerably by region, so a call to prayer heard in Cairo, Istanbul or Jakarta can sound quite distinct in melodic character and vocal style even though the underlying words are effectively identical, reflecting the influence of local musical and recitational traditions on how the fixed text is vocally rendered.
Serving as muezzin has traditionally been a respected role requiring not formal musical training in the Western sense but a strong, clear voice, correct knowledge of Arabic pronunciation, and command of the appropriate melodic conventions, and many mosques historically selected or trained a muezzin specifically for these vocal qualities, some muezzins gaining considerable local or even wider reputation for the beauty of their delivery. The shift from live, unamplified calling from atop the minaret to loudspeaker broadcast over the twentieth century changed the adhan's practical delivery considerably, extending a single muezzin's reach across a far larger area and, in many majority-Muslim cities, creating the now-familiar experience of multiple mosques' calls to prayer overlapping audibly across the same neighborhood.