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A raw audio stream is a constant bit rate stream. If you compress it, it becomes (in general) a variable bit rate stream. The bit rate contains information about the raw audio which survives encryption.

If you compress a document that simply obscures the size of the original document.

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If you compress a document that simply obscures the size of the original document.

That is not generally true - if you compress AAA and ABC then the compressed version also leaks information about the content exactly like in the case of an audio stream and it reveals more information then the lengths of the uncompressed documents which don't differ.

Yes, you can concoct theoretical cases where compressing documents leaks information. But there's a huge difference: compressing a document only leaks a single data point whereas compressing a stream produces a continual information leak. Furthermore, there are common use cases (with audio being the poster child) where compression leaks in streams can be used in practical attacks. I'm not aware of even a single case of a compression leak being used in a real attack on an encrypted document.

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