one-too-many smoke detector cavities?
That silver block with two screw terminals is probably a doorbell transformer, which converts (I'm guessing) 120VAC line voltage to the 24VAC that the doorbell needs.

With that in mind, you cannot wall off boxes that contain line voltages; the contents must be accessible. If a box only contains low voltages, then you can wall it off. However, the doorbell transformer input probably must be accessible, and it looks like you have line voltages in the top box as well, which would mean both must be accessible.