Or, aliens might not use vibrations in gas pressure as a means of communication. Or, aliens might use the entire probe as nutrient, not even realising its significance. Or, they might just consider it a piece of useless space debris and disregard it.
If you take all of the assumptions the designers of the message did - which is basically that the probe will be intercepted by a technologically advanced civilisation that sees in roughly the same EM spectrum we do, uses some kind of writing system, is familiar with basic math and uses some form of audible communication - it stands to reason they'd also be able to decipher the message that tells them how to listen to the record.
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