To test the primary microphone on the bottom of your iPhone, open Voice Memos and tap the record icon. Then speak into the microphone and tap the play icon to play back the recording. You should be able to hear your voice clearly.
To test the front microphone, open the Camera app and record a video using the front camera. To test the rear microphone, open the Camera app and record a video using the back camera. When you play back the video you should hear your voice clearly.
If your voice sounds faint when you’re on a phone call, make sure that your rear microphone isn’t blocked or covered.
If Siri can't hear you, make sure that the receiver and front microphone on top of your iPhone aren't covered or dirty.