It's not Kruglov that the player hears saying "no, i'm not leaving him". It's one of Strelok's group (Ghost, Guide or Doc). The player (Strelok) is having a flashback during the blowout scene while with Kruglov. He hears "No, Fang, I'm not leaving him!". The sound clip specifically and clearly mentions the name Fang.

For those who play the game, once I get this, should I use Stalker Soup, or do a vanilla playthrough with the Zone Reclamation patch first?