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i am pretty sure the old format was AAC in an audible DRM wrapper.
i doubt that would develop their own new format from scratch , so there are not a lot of contenders. MP3 is not really drmable, Microsoft have abandoned WMA, so there's AAC, OGG vorbis ( as used by spotify), what else? Not flac, because that's lossless not "slightly compressed"
i suspect they have just upped the AAC bit rate a little and maybe hardened the DRM,
variable bit rate for speech makes not much sense, complexity does not vary enough.
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