How to retain value of Checkbox checked?

Hi,
 
I have a page which has a graph. The page is Default.aspx. When the user clicks on the graph  the page requested is Default.aspx?index=10.
 
But Page.IsPostback is false in this case when query string is present in the url.
 
The Page also has a checkbox. This checkbox should be checked when the page first loads, but should be  whatever the user selects (viewstate??) on subsequent clicks.
 
 
string indexVal = Request.QueryString["index"];

if (indexVal != null)
{
index = Convert.ToInt32(indexVal);
hdnPlaySound.Value = "false";
}
else
{
index = 0;
hdnPlaySound.Value = ...