,
fajr, maghrib and isha prayer are said loudly. zuhr and asr are said silently. by loudly, it means only when reciting surah fatiha and second surah when standing.
scholars say that zuhr and asr are said silently bcoz its day-time and busy period with noise, etc. fajr, maghrib and isha are said loudly as its peaceful and we can concentrate.
some also say that during beginning of Islam, muslims were not able to pray loudly during day. so to remember their hardships, we follow same.