Restoring to Earlier Version of iPhone Software

Moving to a previous iOS version requires installing a device-specific IPSW file on your iPhone 7 Plus. These come from Apple, and are only available for recent versions of iOS. At present any firmware below 12.2 is no longer signed by Apple and you will not be able to install it. You can confirm this by searching the Internet for "IPSW Downloads."

No what I am saying is you asked two questions.

1. Is there a reliable source of iOS firmware for older versions - the answer is NO.
2. Can I roll back iOS to a previous release - again, the answer is NO.

Beyond that, I have said nothing about your problem or issue other than that it could be software, or it could be hardware. I and nobody else here has any method to discern what it is. You can deal with Apple for an out of warranty replacement, you can buy a new iPhone, or you can roll the dice and try some unknown source’s rogue firmware. That choice is yours to make.

As far as the worst that can happen with the latter choice, is you install a bunch of unknown malware along with the illegal firmware offered. Again, your choice to make.

There is no reliable alternate source of iOS firmware update files. Apple does not authorize anyone to distribute them, so everyone doing so is doing so illegally (it’s proprietary IP and nobody has any rights to distribute it) and thus may also be suspect of altering the installers.

You are assuming that the update killed your iPhone but it actually could be that the audio IC was failing and the update finished it off. That does not mean it is Apple's fault and they should give you a new iPhone. Since you don't trust Apple take it to a competent 3rd party repair shop that can do logic board soldering and have them test the audio IC. If it has failed you can pay them to replace it which will be a lower cost than doing an out-of-warranty replacement of your iPhone with Apple but you may forfeit any future service from Apple on that phone. Shouldn't be a problem for you.

There is no way to downgrade your phone to an older iOS.

I'm not sure if you are just asking rhetorical questions or just venting. If you are not happy with Apple products, you are not being forced to buy them. I'm not sure what you expect from your fellow Apple-product users here.

You’re contention that the iOS update killed your microphone is nothing more than your conjecture. Others have not experienced that, so how do you know, for a fact, that the microphone issue is not simply coincidental? You have no proof at all that iOS is the issue here. You could have a simple hardware failure.

If someone kills your cow, you still have to prove the person you accuse actually killed your cow. Your accusation by itself is not proof the accused party is guilty of anything.

Yes, as your link actually says, it can be a software OR a hardware issue (hence the note that if troubleshooting doesn’t fix it contact Apple). You have still not proven it is a software, or a hardware issue. If it is a hardware issue, contact Apple about an out of warranty replacement (yes, there is a fee, since it is out of warranty).

There have been multiple updates since January, do all of them cause this? How far back do you have to roll back to fix it?

You could wait until the next update to see if your theory is correct.

Certainly an option.