how to edit voice sound like speak inside head

[Vivif]

what effect could I use to make it sound like someone's talking to themselves, in their head

like in this video

at 0:55 to 1:04

https://www.youtube.com/watch?v=9KSGUVcNfZM&t=64s

[SteveG_AudioMasters_]

Reverberation. There are several in Audition - take your pick!

[Vivif]

i new to adobe audition, can you help me in detail?

i dont know what is Reverberation.

[SteveG_AudioMasters_]

There is loads of information about this available with a simple Google search:

Google search about applying reverberation in Audition