Well, I would treat the idea that it is indeed a soft spot with a little skepticism until you get some more information. It would be uncommon for the hull to be cored. I would expect it to be solid fiberglass. So, the tapping sound difference would be due to something else.
That said, I ran across this:
https://sailingtexas.com/201901/shighlander20109.html
The owner talks about an area of balsa core behind the swing keel trunk that needs repair. So, maybe.
More information is needed, and ultimately, you might need to bore a small hole to inspect (though that would not be my next move). But, if the core is indeed wet, that is a fairly involved repair. I have no experience with such a repair in the hull because, as I said, it is typically all-fiberglass down there - at least on boats I have seen or worked on.
Elsewhere (like on the deck), such a repair often involves removing one layer of fiberglass, revealing the damaged core but leaving the other fiberglass on the other side intact. Then, clean out and replace the damaged core, replace the removed fiberglass, sand, etc.
To be clear - I do not know the preferred way to make such a repair in the hull. I would think that it would be preferable to remove the fiberglass on the inside, likely being thinner and less fussy cosmetically. But, access inside is likely an issue. There will be someone here that has more experience with this than I do.
But ... again ... I would do a little more investigation to ensure that is indeed the problem. A moisture meter could help. The next step might be to drill a small (1/4") hole in the middle of the area, being careful to catch whatever comes out of the hole. Water might just drip out, and that would tell you something. But, it should also give you some idea of what else is in there and whether it is wet/damaged. If you do this, take some care to pay attention to what is on the other side of the hole before you drill. A hole that size is easily repaired.
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