E.B.
My daughter has a sleep disorder. She has been through several sleep tests and clinics. So if you will allow me to offer some suggestions:
A terrible audio book will not produce healthy sleep. A terribly written book may be boring, but being bored doesn't produce restful sleep. Sure, a lot of people can doze off when listening to a boring speech or while watching a really bad movie, but that's not the kind of restorative sleep pattern that should occur during the night.
Sleep specialists have procedures that they call "sleep hygiene". Sleep hygiene consists of certain practices to follow before bedtime. They include: limiting alcohol and caffeine drinks (stop drinking coffee, tea, alcohol, energy drinks about 2 hours or more before bedtime; drink cool water or a soothing caffeine-free tea that is good for relaxation); eliminating all screen time an hour before bed (screens emit a blue light that we don't really perceive that stimulates the brain, so even reading a book of calming poetry on an e-reader isn't helpful); practicing relaxation breathing (lots of examples online); making sure the bedroom is cool and quiet and dark; and using the bed for sleep only (and not for lying on to watch tv for example).
Then, a white noise machine can be helpful. There are sound machines online that can produce brown or pink noise too. Those are deeper, more complex sounds. Plenty available very inexpensively on Amazon. We use a pink noise machine.
There are CDs available that help people feel rested and fall asleep. A great source is the Mayo Clinic. Just google Mayo Clinic sleep cd and you can purchase one online. These sort of CDs don't just play soothing music. They contain "guided" sleep techniques, where a trained sleep specialist guides the person through a series of steps, and breathing, etc. Then there will be specialized musical tones that are created, well, there's a whole bunch of sleep science there that is kind of beyond me, but the tones really do help.
If the person you're writing about really is experiencing sleep problems that are affecting his or her life and health, consider consulting a sleep specialist. Major hospitals have sleep clinics. Don't mess around with something as important as good sleep by buying a terrible audio book.