If the sub is sitting bellow 1,000m and the hull has not been compromised, the changes of rescue for any surviving crew are virtually nil. The Brazilian Navy's rescue equipment can do 300m and the best the Americans and the British have can do at most 600m (if that).
The sub's last reported position had the average depth of 1,000m. Considering that the max length of the cable that holds the SEPIRB (Submarine Emergency Position Indicating Radio Beacon) is 900m, that might explain why we are not getting any emergency signals (the transmission buoy is 100m under water).
The prospects are not good.