asterisk users - May 2007 - using voip software client as public address system. Low volume

Hello all.

We have an asterisk working perfectly but we need a sollution for the PA system.
Before Asterisk PBX we had an expensive analog PBX which plugged an
extension into an audio amplifier, and that was the PA system.

Now, the Asterisk server is quite far from the audio amplifier and it
has no audio card. So my idea is to plug the audio card of another
linux server, which is over the amplifier, into the amplifier.
I've configured a pjsua with auto answer but the audio is very poor,
very low volume compared to a normal audio playing (like 'aplay
ttt.wav').
Is there any way to increase the volume of sip calls?
Is a client side configuration, a server side or both :)
Any ideas?

Please, I'm going mad.

Thanks in advance.