Problem with voice

I config a server for Asterisk 1.6.2 on the Debian.i set 1 hard phone( Grandstream bt200) and a laptop with soft phone (SFLphone ).
however,my problem is when communication between soft phone and hard phone voice is low.
i tested with other soft phone .problem not resolve.even at communication between two hard phone(Grandstream bt200) voice is low.
how can i resolve this problem?

I am quite confident that this problem has nothing to do with Asterisk at all. Try to tweak the gains and volumes on the phone and softphone (or the OS sound settings).