If you want Openbox to invoke multiple commands for a keybinding, you can just add extra action elements inside the keybind element. Sometimes XMMS freezes up on my machine, so I use this in my rc.xml file:

<keybind key="C-A-k">
  <action name="Execute"><execute>killall -9 xmms</execute></action>
  <action name="Execute"><execute>xmms</execute></action>
</keybind>

This has been tested with Openbox 3.2.