when the first member creates and enters in a new conference

everything is ok. but if a new memeber enters there is no audio

in the conference, unless the ivr.

I have a very simple conference dialplan like this

        <extension name="create_conference">
                <condition field="${sip_to_uri}" expression="^000(\d{10,15})@$${domain}$">
                        <action application="set" data="instant_ringback=true"/>
                        <action application="ring_ready"/>
                        <action application="sleep" data="3000"/>
                        <action application="answer"/>
                        <action application="conference" data="$1-${domain_name}@wideband"/>
                </condition>
        </extension>

I tried to add

<action application="export" data="absolute_codec_string=speex@16000k"/>

but no success

Thanks