I got a better solution. 
<keybind key="XF86AudioLowerVolume">
<action name="Execute">
<execute>amixer -q set Master 10-</execute>
</action>
</keybind>
<keybind key="XF86AudioRaiseVolume">
<action name="Execute">
<execute>amixer -q set Master 10+/execute>
</action>
</keybind>
<keybind key="XF86AudioMute">
<action name="Execute">
<execute>amixer -q set Master toggle</execute>
</action>
</keybind>Then toggling mute works.