Seeing that I had been looking for an answer to this very question recently, I came across this
Amazon review.
At first it didn't make sense to me why I'd want to attach a mount to a mount but then I dawned on me that you'd mount the
Bell box cover to a proper electrical box that's used to router your wiring through. The circular nut that come with the Bell box cover is hollow, allowing the speaker wire to pass through it.
Another benefit of using the Bell box cover is an increased range of motion for tilting the speaker.
Be mindful however that while the Bell box cover does allow you to mount the speaker in a horizontal fashion, it does not offer any tilt functionality. So you're nowhere with this as a solution if your needs are to mount the speaker in a horizontal manner.