integer isSoundTurnedOn;
default
{
    touch_start(integer total_number)
    {
//      toggle isSoundTurnedOn between TRUE and FALSE
        isSoundTurnedOn = !isSoundTurnedOn;

        if (isSoundTurnedOn)
        {
           //play sound
        }
        else
        {
           // stop sound
        }
    }
}