That is the option for the audio itself. The transcripts of the audio (you do know what transcripts are, right?) are always sent to apple as per their privacy policy.
"When you use Siri, your device will indicate in Siri Settings if the things you say are processed on your device and not sent to Siri servers. Otherwise, your voice inputs are sent to and processed on Siri servers. In all cases, transcripts of your interactions will be sent to Apple to process your requests."
It's pretty clear and not in dispute that your transcripts are always sent to Apple.
That’s because Siri doesn’t run on-device - phones like the iPhone 6 can’t run that level of analysis. They “collect transcripts” insofar as they need to process your request.
Nonetheless, Siri is trivial to disable altogether.
Yeah if you asked the average person on the sheet (e.g. you) if they thought Siri was 100% private, they'd say yes because Apple has mislead them and said it is. That's the point. Apple says everything is private but then secretly collects data via their privacy policies.
"When you use Siri, your device will indicate in Siri Settings if the things you say are processed on your device and not sent to Siri servers. Otherwise, your voice inputs are sent to and processed on Siri servers. In all cases, transcripts of your interactions will be sent to Apple to process your requests."
It's pretty clear and not in dispute that your transcripts are always sent to Apple.