Cho'gall is in the Northern Fel Spire. Here is his dialogue upon entering the room and then freeing him:
Cho'gall says: You...outsider. Destory that relic, and set us free. (Crush the relic! Smash the relic!) <--FYI, the part in parenthesis is his second head speaking.
Cho'gall says: Seek out Gul'dan if you truly wish to stop the Iron Horde. (Gul'dan! Gul'dan powers the portal!)
Teron'gor is in the Southern Fel Spire. Here is his dialogue upon entering the room and then freeing him:
Teron'gor says: You...strange invaders...
Teron'gor says: You seek to disable the Dark Portal?
Teron'gor says: I can help you. Just...set me free.
Teron'gor says: Those Iron cowards were using us as conduits to power their portal. They will regret that.
Before you enter each spire, the two dwarves among the NPCs blow the doors to each one, and as you pass, they say a couple of voice acted lines:
Hansel Heavyhands says: Get on in there, champ!
Thaelin Darkanvil says: Don't worry, <Name>. We've got your back!
After disabling one spire, Khadgar lets you know it's doing something:
Archmage Khadgar yells: Look! The portal grows weaker!
Archmage Khadgar yells: Hold that front! Our work is nearly done!