.wav files are RIFF containers of type 'WAVE'. These files can contain many different types of RIFF chunks, but the required chunks are a 'fmt ' (format information) and 'data' (audio payload). The format chunk describes the encoding of the audio payload data, among other information (channel count, sample rate).[0]

Although .wav files are, today, typically used for non-compressed PCM data (WAVE_FORMAT_PCM), even the original 1991 RIFF specification allowed for three compressed formats: mu-law, a-law, ADPCM.[1] These are all efficient to compute and I don't find it completely implausible that such low quality compression would be used. Modern .wav files may use the WAVEFORMATEX or WAVEFORMATEXTENSIBLE chunk, which uses GUIDs to identify formats. It supports the original compressed WAVE formats,[2] but also more modern compressed formats. Here is For example, here is Microsoft's list of sub-format GUIDs (includes MPEG formats and AC-3):

https://learn.microsoft.com/en-us/windows-hardware/drivers/a...

[1] https://www.aelius.com/njh/wavemetatools/doc/riffmci.pdf heading "WAVE Format Categories".

[2] https://learn.microsoft.com/en-us/windows-hardware/drivers/d...

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