mixMonitor problem

1.problem:
when call(such as 9002 call 89003,see detail below),a .wav file created under /var/spool/asterisk/monitor,but the file always has the length 44 bit,and can not be opened by realplay or windows media player.
why?could anyone give some advice?

2.enviroment:
1)Asterisk:1.4.0
2)extensions.conf
exten => 8.,1,Set(CALLFILENAME=${CALLERID(num)}${EXTEN:1}_${STRFTIME(${EPOCH},%Y-%m-%d %H:%M:%S)})

exten => _8.,2,MixMonitor(${CALLFILENAME}.wav)
exten => _8.,3,Dial(SIP/${EXTEN:1})

exten =>9001,1,Dial(SIP/9001,wW)
exten =>9002,1,Dial(SIP/9002,wW)
exten =>9003,1,Dial(SIP/9003,wW)