I am on the same situation, I got approved more than 6 weeks ago but the problem is I haven't wrote my apartment number in the application, so when the postman comes to the building and he finds no apartment number on the letter, he returns it to the post.
I contacted CIC, changed my address and they said they will leave a note to the post office to transfer my letter to the new address, 10 days passed and haven't received anything. Contacted them yesterday and they said there is no delay or processing time for this, the office should process the letter transfer as soon as they read the note.
My current study permit expires in 20 days, however, you should not worry because you were already accepted an extension so you are legally in Canada eventho the one you have is expired. The problem is I can't do the college reg. for the next semester since I haven't received the new SP.
Call CIC around 7:45 am ( Canada Time ) or eventho if they say that they have a high volume of calls, just continue trying, they contacted me to an agent after several times.
Goodluck and do not worry