Welcome to the Apple Support Communities!
I understand that your iPhone is experiencing sound issues with the bottom microphone with several apps including Snapchat, Instagram, and the built in Voice Memos. I’d be happy to help provide guidance on the issues your iPhone has been experiencing.
Based on your question, I understand that you would like to narrow down this issue to either hardware or software. So far, the troubleshooting you have performed has been unsuccessful in resolving the issue.
To continue the troubleshooting process, please read over and work through the troubleshooting information located in the following article:
Get help with the microphones on your iPhone, iPad, and iPod touch - Apple Support
If you still have issues after testing the microphone on your device, try these steps:
- Unplug any accessory plugged into the headset jack.
- Avoid blocking the microphone and iSight mic with your fingers while holding the device or recording video.
- Remove screen protectors, films, or cases that cover the microphone, iSight mic, or iPhone receiver.
- Clear any debris from the microphones or iPhone receiver. Refer to the user guide for your device on microphone locations.
- Restart your device.
- Make sure you have the latest version of iOS.
If you've tried these steps and people still can't hear you on phone or FaceTime calls or Siri doesn’t hear your requests, contact Apple Support.
FaceTime is not available in all countries or regions.
Have a great day!