An unofficial companion piece to last year’s micro-budgeted “Another Earth’’ (both premiered at Sundance 2011), Zal Batmanglij’s tight little film also stars co-writer and co-producer Brit Marling.

This time the enigmatic blonde plays a Los Angeles cult leader who claims she’s an ailing visitor — complete with oxygen tank — from the post-apocalyptic world of 2054.

She quickly starts developing doubts about the cult’s two latest recruits (Christopher Denham, Nicole Vicius), who are actually plotting to smuggle a camera and recording equipment into her secret meetings.

The newcomers are subjected to an increasingly intense series of loyalty tests, progressing from vomiting on demand to child kidnapping.

“Sound of My Voice’’ holds your attention for a while, but fails to build much suspense as it races toward a predictable climax. It probably would have worked better as a series of Webisodes, which reportedly was the original plan.