An update:
I finally found the time to remove the headboard. There is no room to recess a speaker. The aft wall of the chain locker is directly behind the headboard. Best I’d be able to do would be to raise the factory supplied speakers a bit higher which might help us avoid hitting our heads when sitting up in the berth to read or watch tv.
As an aside, I recently found the drain in the chain locker to be blocked. I struggled, using an air compressor and high pressure water, to clear the clog. It took some time, but it finally worked. Well today, while investigating the speaker issue, I found a cutout behind the removable berth cushion on the forward wall that gives almost direct access to that drain’s hose and the hose clamps inside the hull that secure it to the drain outlet in the bow. Would have made clearing the clog a whole lot easier had I known. You live and learn.