Hi, interesting. As I recall (possibly flawed by the huge passage of time ) there was an issue with this with early televison commercials. Both audio and video. A subliminal message was placed in the audio track suggesting that you buy the product. Similarily a single video screen was stuck in some commercials saying the same thing. A single frame was not obviously noticed by the viewer..but the brain made note of it. There was a lot of uproar and the practice was stopped. I don't know if it was made illegal or just that the comapnies that did it were sued etc. An early attempt at mind control.
Now that was sneaky. Commercials have those loud sounds so no one would notice if there were some subtle background music. Sounds are just that important cause they can easily change how you perceive stuff. Hearing this furthers my opinion that I wouldn't like an audiobook with background music.