Will change of logic board and home button fix the microphone issue?

First,have you tried a different mic? Does the phone have no mic at all whatsoever,or is it just Siri that doesn’t work for example? Does sound work?

audio disease is a “thing” in the iPhone 7. The audio chip(soldered to the board balled grid array) commonly has a joint break loose due to the phone getting bent and flexed over time(not necessarily noticeable bending). The audio ic is in a bad spot for this and there’s one specific pad that breaks. This requires lots of skill and tools to replace. But this can cause slow boot issues,no mic or sound problems,etc.

But of course you’d need to try known good housing(everything known good other than the logic board) to know for sure.

if you replace the logic board and home button,of course it WOULD work fine in the new phone. But this is if the problem isn’t with the mics in the phone. But swapping logic boards and bringing the paired button over to the other phone casing will have no problem. But the cost of a logic board and home button sometimes is about the same price as a whole used 7 plus. Remember though,if you remove the logic board,be absolutely positively sure to NOT mix up the screws. Long screw damage is a thing. It happens so often.

Let me know how it goes.