Notification sound ( Message,yahoo, email etc) working for me after 6.1.1.b.1.75 update. As per previous discussion topics, I realized that notification sound will disappear once we received balance amount notification from our mobile operator ( Prepaid card ). I think postpaid users don't have this issue so far.
After the update, I have turned default notification tone to silent and I kept on for other apps ( Message,yahoo, email etc). It's already 2 days after update and I have attended many calls too, but, notification tone still working. Try this method. Will feedback you all if my notification sound stops working.