I have never used PushSharp, but the normal way of adding sounds to Push notification is to simple specify the full filename, as you do. Are you sure the sound name is correct and the file is available in the app?
You can try scheduling a local push notification with the sound by calling a method like:
+(void)scheduleNotificationForDate:(NSDate*)date withMessage:(NSString*)message andOKButtonTitle:(NSString*)buttonTitle andPayLoad:(NSMutableDictionary*)dic andSoundName:(NSString*)soundName
{
UILocalNotification *not =[[UILocalNotification alloc] init];
not.fireDate = date;
not.alertBody = message;
not.alertAction = buttonTitle;
not.soundName = soundName;
[[UIApplication sharedApplication] scheduleLocalNotification:not];
}