Only the text survives
The device uploads a recording, confirms the text came back, and then deletes the audio from its own card; the transcription server deletes its copy once it is done. What you are left with is a transcript you can search, not an archive you have to guard.
Your transcripts, your storage
Transcripts can save straight into your own Google Drive. We ask only for access to the files we create there, never permission to read the rest of your drive. If you would rather not use ours at all, you can point the device at a transcription service you run yourself.
Encrypted on the card
Audio is written to the MicroSD card with AES-256, using a key derived from a secret stored in the device at the factory. Take the card out and put it in a reader and you get ciphertext; only the transcription server can decrypt it, to transcribe it. This protects a card that walks off; it is not designed to stop someone who opens the device.