It’s probably because the file is stereo. That’s correct, RapidEars doesn’t work on a single pause-bounded utterance in the sense that stock OpenEars does, so it doesn’t have a method for outputting that complete utterance as a WAV.
I construct replicable tests for RapidEars by making a recording and playing it out of a speaker and into the device, cued by a note in the console that says “press play now”. While it is not the most deterministic thing going, it is the most informative approach I can think of that actually replicates real-world behavior without interfering with RapidEars’ resource management at the same time I’m trying to test it.