There is no need of very hi-fi sound system for public address. some time people select stereo/surround sound/5.1 for small auditorium which is of no advantage and expensive.
I select following sound system for 50 feet x 50 feet x 15 feet height Hall ( Gypsum celling for acoustic purpose). 120 students sitting Capacity.
1. 120W Amplifier (Ahuja Make) 5 + 2 Aux in.
2. 32W x 4 Nos of speakers.
3. Wireless Microphone.
Speaker should face opposite Microphone direction otherwise echo effect.