I would recommend taking to the Apple store, as it seems like a hardware issue. The camera app (while using the back camera) will use the mic on the back of the phone, while most other apps will use the front mic on the bottom of the phone. Since your back mic seems to be working, it seems like the front mic has failed. I bought an 8+ at launch and noticed the speakers were getting pretty quiet, so I took it to the Apple store to run some tests. When the genius did the audio test, the only mic that worked was the back mic, and both the speakers failed. I hadn’t noticed the mics being broken as I almost never use the front mic. Luckily, cleaning the speakers out fixed them, but the front mic was still damaged. The problem with replacing the front mic is that it’s part of the display assembly (at least according to the genius that was helping me), so replacing it requires a new screen. They did fix it for free though, so you may be able to get a new screen for free