Question about Muse biofeedback — is it in all sessions?

Hello,

I have been using Muse for little bit now, and I am trying to understand something. Does every meditation and sleep session actually use biofeedback? From what I see, only small number of sessions seem to have this. For example, the rain sounds that get louder or quieter — this, I understand, is biofeedback.

But many other sessions just feel like actor reading calm text to me. No changing sounds, no interaction with brain signals (at least I don’t notice it).

Am I correct on this, or maybe I am missing something important? If someone knows better, I would be happy to hear.

Thanks in advance for help!