There are two sound files in  /var/spool/asterisk/monitor 
a-in.gsm
a-out.gsm

when the call hangs up I use

exten => h,1,system(sox /var/spool/asterisk/monitor/a-in.gsm /var/spool/asterisk/monitor/a-out.gsm /var/spool/asterisk/monitor/output.gsm)

it doesnt create output.gsm file in /var/spool/asterisk/monitor/ but when I use the same command at Linux SHELL. It works!! .. 
I have tried removing the Paths (/var/spool/asterisk/monitor/) from System() but no success.
Just to check I tried to wrong file names ...for example: a-innnn.gsm instead of a-in.gsm, Asterisk dont show any error msg at CLI when it executes SYSTEM().
I tried a very simple command MKDIR in System() just to check it SYSTEM() works....
that command works......if everything is working in SYSTEM() then why not "SOX"??

Anyhelp will be highly appreciated.