So I've not gotten to Leviathan yet (waiting to play it just before/after Thessia) but I got to the second dream, just after Tuchanka, and I got the fright of my life.
Clear voices in the dream. Ashley, whose dead on this playthrough, speaking the lines she did before she died on Virmire, and Mordin speaking too. I've always heard whispers and vague voices, but these were incredibly clear and I could make out every word, they were even much louder than the whipsers.
Then afterwards, instead of going "I miss Ashley" Shepard says he misses some friends that he lost during the war, and Liara asks me if I'm thinking of anyone in particular. I've only got a choice between "I miss Ashley" or "I'll let them rest", I need to replay this on a playthrough with other squaddies dead because right now I'm doing a perfect playthrough.
I assume this was added in with the EC, it's kinda cool.