I just have a laptop and pc speakers (sony and bose for pc).
They are not loud enough for 300 seats hall.
What do I have to do to make loud sound system with laptop ? I don't know what I need (??soundcard, big woofer, amplifier, mixer?? as like old analog system?)
I have budget of $500.
Your existing PC already has everything you need to send the audio signal. You will probably just need to pick up a 1/8 mini, to RCA adapter.
Depending on your needs, you are going to be hard pressed to find a music system loud enough system for $500.
Everything you need is in the box. You will probably have to supply speaker cables, and the above mentioned cable connection.
Be mindful, that high volumes on low current equipment is going to cause all sorts of nasty audio quality. You want to keep any of this lower budget equipment at lower volumes.