He can't in the novel, he can interpret her bawking/inner voice when she was a "chicken". 

In the drama they show her inner monologue alot so everyone seems to pause in those scenes, so it feels like he can. Not verified in the drama so I can't be 100% certain if they are trying for that or not.

 knitmergetanklock:

N.B I'm only up to episode 24

When she is a "chicken" it seems like  X can "hear" her thoughts.

We get a lot of her interior monologue ( which I love) when SL goes back to the Immortal /Divine Realm. Can Ancient God X still hear her thoughts? Can he hear everyone ?

 

To answer your question, he will no longer hear her thoughts when she goes back to the immortal realm. And he can't hear everyone's thoughts. Your welcome!