I've been basically fiddling with this myself trying to find the perfect mixture for flashbacks and whatnot. While I can't pinpoint exactly what it is, for sure try reversing the audio and adding reverb, then you reverse it again (so that it plays normally) and add a delay effect.
That's the very basics.
If you're using Audacity, this video covers a few different effects you can achieve and in the description it discusses how to get them.
https://www.youtube.com/watch?v=yP0QActU5s4
Hope this is helpful to you. =D