As is common with most phones, there are two sets of microphones on yours; the lower microphone is used when you're talking on a phone call and the upper microphone is for most other uses.
You can test this out by making a phone call. During a normal call it appears your lower microphone isn't working; can they hear you if you switch to speakerphone? If so, that would be a pretty definite indication that there's an issue with your lower microphone.
First thing to do is to clean the opening for the microphone, especially if you carry your phone in your pocket, as it can easily collect dust, pocket lint or debris. There are several ways to do this including rubbing alcohol, toothpicks, vacuum and compressed air.
If you don't have any luck with cleaning, then it may be time to replace the microphone. On your phone the lower microphone is mounted on the lightning connector flex cable assembly. Nobody tries to replace just the microphone; instead you'll be replacing the cable assembly. Here's the part that iFixit sells.
[product|IF240-048|iPhone 14 Plus Lightning Connector Assembly]
You can see where the microphone is in this picture; I added an arrow pointing to it.
[image|3622954]
iFixit doesn't have a guide to that repair, but the Spanish site Nadie Me Llama Gallina has just what you need. Of course it's in Spanish but if you use it with the Chrome browser's translation feature, it becomes quite usable no matter what your native language is.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/apple-5/apple-iphone-14-plus-1379/flex-de-carga-12475|Manuals / Apple iPhone 14 Plus / Charging Flex Cable - Nobody Calls Me Chicken]
As a side note just for the sake of completeness, the upper microphone is located on the sensor cable assembly that's mounted on the screen. Here's a picture showing where the microphone is located.
[image|3622962]
Here's the guide you'd need to use to replace it; note that this is a screen replacement guide, but if you follow it exactly and swap out the sensor assembly instead of the screen, it'll accomplish exactly what you need to do.
[guide|153338|iPhone 14 Plus Screen Replacement - iFixit Repair Guide]