Earpiece speaker doesn't work - NOT flex cable.

I have an iPhone 7 Plus that has never had previous repair. All Apple OEM parts remain intact. There are no signs of liquid damage or fall damage to the phone. Customer says that they have taken care of the phone during the entire time they've owned it until one day, the earpiece speaker stopped working.

This has been a problem that has played out with this particular customer over about 3 months. When they initially came to me, a simple forced restart (Power + Vol - ) restored the earpiece speaker. Some weeks later, they returned with the same problem. Another forced restart solved it.

A few more weeks go by and they returned again, although this time, a forced restart did not solve it. Judging by how a restart seemed to clear up the problem in the past, I suggested we perform am iTunes backup and DFU mode restore. This fixed the problem this time.

The customer returned to me yesterday, same problem. This time, I suggested replacing the front camera flex cable and/or earpiece speaker. I removed the screen (being sure to disconnect the battery cable first) and replaced both the earpiece speaker and front camera flex cable as I have done dozens and dozens of times now. The black adhesive was covering the gold contact pads. This did not fix the problem. No earpiece speaker.

I went through my entire stock of front camera flex cables, one by one, as well as my earpiece speakers. None of them restored the earpiece speaker.

I then changed the LCD assembly TWICE, trying both the original front camera flex cable with the new LCD assembly and a new front camera flex cable + earpiece speaker with the new LCD assembly, and no combination of the two restores the earpiece speaker.

Just so happens, another customer brought in an iPhone 7 Plus that received some drop and liquid damage to the screen and front camera/earpiece speaker area. I used some of the same front camera flex cables that did not work on my original customer's phone and on this new customer's phone, they worked immediately, as they have with literally every other iPhone 7 Plus that has come into my shop.

I have tried cleaning the FPC connectors on the main logic board, but that didn't produce any results.

I think I should also mention that the phone takes longer to boot than usual, but once it has booted, it operates normally in terms of touch responsiveness. In Voice Recorder, the option to record audio is greyed out. Also, the speaker will randomly begin to work again for no apparent reason after being left alone for some time. As of this writing, the speaker is currently working. If I were to reboot the phone, it will stop working for some unknown amount of time before it will randomly begin working again. Very strange.

I'm beginning to suspect audio IC, but I am not certain. Anything else I can test for/try? I'm kind of stumped.

Thanks.