Switch soundinterface

Please I made a buttons together with Krpano buttons style and I put his action on skin xml together with all actions.
The problem is that I have to push two times the button to work (Mute) and two time to resume . What am I doing wrong?
Thank you

Ok the code is

<plugin name="somOn" devices="desktop" align="bottom" x="+160" y="10" style="button" crop="440|0|40|40" onovercrop="440|40|40|40" ondowncrop="440|80|40|40" onhover="showtext(Som, BUTTONSTYLE);" onclick="set(plugin[soundinterface].mute,true); set(plugin[somOn].visible,false); set(plugin[somOff].visible,true);" visible="" />
<plugin name="somOff" devices="desktop" align="bottom" x="+160" y="10" style="button" crop="480|0|40|40" onovercrop="480|40|40|40" ondowncrop="480|80|40|40" onhover="showtext(Som, BUTTONSTYLE);" onclick="set(plugin[soundinterface].mute,false); set(plugin[somOff].visible,false); set(plugin[somOn].visible,true);" visible="" />

I dont want put the sound name because always i have to change the skin xml.

Thank you