It might be that you have to edit Planeshift configuration files manually to disable sound.
Find a file called psclient.cfg, open it with Notepad, then look for the following line (the end may vary and can be .software, .ds3d or .openal):
System.PlugIns.iSoundRender = crystalspace.sound.render.software
Add a semicolon ; to the beginning of this line:
;System.PlugIns.iSoundRender = crystalspace.sound.render.software
Then save the configuration file.