Playback and Background Problem

Hi:
My problem is with my analog line. I have one FXO line (X100P). The file configuration zapata.conf is:

[code][channels]
; Keywords
context = pstn
;switchtype = national
signalling = fxs_ks

; Analog Call Progress
busydetect = yes

; Timing Parameters
;rxwink = 300

; Caller ID Options
usecallerid = yes
hidecallerid = no
callwaitingcallerid = yes

; Other Call Feature Options
threewaycalling = yes
cancallforward = yes
transfer = yes
immediate = no
callwaiting = yes
callreturn = yes
pulsedial = yes
usecallingpres = yes

; Audio Quiality Tuning Options
echocancel = yes
echocancelwhenbridged = yes
rxgain = 3.0
txgain = 8.0

; Group Call Options
group = 1
pickupgroup = 1
callgroup = 1

; Miscellaneous Options
language = en
musiconhold = default
busycount = 6
canpark = yes

channel => 32[/code]

And extensions.conf is:

[pstn] exten => s,1,Answer() exten => s,2,Playback(hello-world) exten => s,3,Hangup()

The problem is the Playback application, the file hello-world.gsm exist and when call, the out is:

-- Executing Answer("Zap/32-1", "") in new stack -- Executing Playback("Zap/32-1", "hello-world") in new stack -- Playing '/var/lib/asterisk/sounds/hello-world' (language 'en')

but don’t play nothing. I test with *.ulaw and *.alaw files but nothing.

The command
exten => s,n,MP3Player(/var/lib/asterisk/mohmp3/sultan.mp3)
work perfect!!!

I need help. Thanks