Following the examples of Contact, The Fountain and more recently I Origins (also starring Brit Marling), Sound of My Voice feeds the North-American movie genre of Cartesianism being challenged by the human experience.
Specifics to Sound of My Voice are its very low budget ($135,000) and its open-ending.
Peter is a school teacher substitute. His life partner is Lorna, an aspiring writer and photographer. As a child, Peter lost his mother to cancer due to her being a member of a healing sect. The couple's project is to infiltrate a small futurologist cult and make a movie to expose the manipulation and the fraud. And may be even prevent a collective suicide.
It turns out the secret cult leader is Maggie, a seductive young blond woman with weak health but much charisma. Thus, as time passes, Lorna starts to fear Maggie as a rival because lies just make her look more fabulous.
It's not about truth. It's about seduction.
If you are a female spectator chances are that you will feel how Peter is actually attracted by Maggie. So, like Lorna, you would probably collaborate with the police to put an end to the story. Conversely if you are a male spectator you will eventually be seduced by Maggie. Then what Maggie says and what Maggie demands seems neither as treacherous or dangerous as first expected. After all, not every beautiful mythomaniac must be a criminal.
No special FX needed : the more you will be impressed by Maggie (Brit Marling's performance) the more you will feel a sort of reality distortion experience.