\\Slight modification to the callback
:sonique.players.default.RegisterCallback("SongStarted","ChangeBG();:sonique.slideenabled=true;",this);

<script name=OnClick>  \\the click script for the enable/disable button
if(:sonique.slideenabled==true){:sonique.players.default.UnregisterCallback("SongStarted");:sonique.slideenabled=false;}
else{:sonique.players.default.RegisterCallback("SongStarted","ChangeBG();:sonique.slideenabled=true;",this);}
</script>