1 Malvina took Verena's body
Malvina took Verena's body and came back to life. When Verena saw herself buried in the wall, she was not dreaming. It was the time where the exchange process of bodies took place. Some people suggest it was a partial take over and not a full one.
2 Malvina became a part of Verena
Some people suggest that Malvina did not posses Verena entirely, but instead she became a part of her and the consciousness of both existed in the same body. This is supported by Verena's final words where she said that she can hardly remember what happened before she came here.
3 Playing the piano was a clue
In the beginning of the movie, Verena said that she is not a good pianist. At the end of the movie, Verena was playing the piano very well because that wasn't her; it was Malvina inside her body.
4 Jacob recognized his mother
Jacob was cured and started talking when he recognized his mother in the body of Verena. When Jacob said 'I missed my mom' in the end of the movie, he was referring to his mom who was inside Verena's body. (See Why are children used in horror films?)
5 Verena was being prepared
Some people suggest that Verena was being prepared for the final event where Malvina will get inside her body. Verena was made to be more like Malvina throughout the movie for the process to happen easily.
6 The mansion was haunted
The mansion was haunted by the spirit of Lilia, Malvina and the other people buried in the tombs. The voices Jacob was listening to were real. This is also why Verena used to see Lilia who was actually dead. (See Dead Silence Movie Ending Explained )
7 Jacob was waiting for his mother's spirit
Jacob knew beforehand that a new woman will come and that his mother's soul is going to be inside her. His mother told him earlier in the movie that another woman will come and then he will know when to speak.