Try
Added selected="selected"
to On so that it is selected when it page loads.
By default always first option is selected
<option selceted="selected" value="on">On</option>
<form id="sound-switch">
<select name="flip-3" id="snd-switch" data-role="flipswitch" data-mini="true">
<option value="off">Off</option>
<option selected="selected" value="on">On</option>
</select>
</form>
Check here http://validator.w3.org/check
SELECTED
is not a member of a group specified for any attribute
using SELECTED
will work but not pass w3 says invalid :)
use selected="selected"