it says here on its imdb page that its "followed by The East (2013)"

I saw The East, and I just watched Sound of my Voice and Brit Marling's character was named Sarah in The East, but it didn't seem to be set in the future and besides having the name they accused Maggie of being I didn't see any connections with the movies. There was no mention of time travel, and I don't think they mentioned Abigail or anything about her mother. I don't think it matters, but I saw The East sometime before I saw Sound of my Voice. Maybe I missed something, but The East also ended on a cliff hanger, but not nearly as big as the one at the end of Sound of my Voice.

Okay. This is just an idea, which may or may not relate to this (likely not an actual connection), but:

Abigail Pritchett is a bit of an outcast (as seen in the two scenes that establish her as different in the film). The former scene suggests her tendency to sleep or nap at odd times (there is mention of narcolepsy, even) and gave me an odd feeling that it might be related to the condition that Maggie has in the film (possibly subtly establishing a bond between them way before the reveal towards the end). The latter scene is the altercation that Peter intervenes in. We see a backpack with the word "TERRORIST" scrawled on it, strongly suggesting that Abigail did it. If that is the case, it gives off this vibe that Abigail feels different from the others, possibly because she is (and excuse the use of this expression) "vibrating on a different frequency," which will definitely be the case of the people you find in the film "The East" a little later. None of these are conclusive, but they make you think.

On a side note, Brit Marling has stated that the two scripts were conceived at the same time and that there definitely is a "crossover" of ideas and ideology between the two (most notably the idea of a "group" or "cult" and the infiltration of such a group).

Furthermore, she has stated along with Zal Batmanglij, who has worked with her on a slew of films that are "out of the ordinary" like "The Recordist" (2007), "Another Earth" (2011), and "Sound of My Voice" (2011), just before "The East" (2013), that "Sound of My Voice" is intended as the first in a trilogy of films that follow Maggie's story. No mention of whether Maggie is who she claims she is, whether they have decided or even know themselves as writers, or if any of those films is actually connected to each other.

Lastly, I think another "clue" (if this turned out to be a part of a longer multi-part film storyline) would be the structures that Abigail is building in her room out of black blocks, as well as possibly the hat that she supposedly hardly ever takes off (but removes that covering when Maggie removes her own during their meeting).

In no way conclusive, but definitely a little deeper than many of the eye-candy special effects sci-fi fare that we are treated to these days (although that has it's place, entertainment-wise, that is).

Personally, I would really appreciate more about Maggie from Brit Marling and Zal Batmanglij. It would have all the makings of a non-mainstream cult-following (pun intended) sci-fi storyline that might stand the test of time on the strengths of its writing and acting alone.

Sort of reminds me of how I was floored by Shane Carruth's "Primer" (2004) and how it used the same kinds of elements to round of possibly the best ever intellectual sci-fi which doesn't rely on special effects to coax forth some goosebumps. (Interesting side-note: There was only one (1) special effects scene in "Primer" (2004) and it has nothing to do with the science or sci-fi aspect of the film itself).

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