which part should I replace

Hi Kokki,

Okay, your iPhone 13 has a couple of microphones but the one you're interested in is the lower one that's used during phone calls. That one is mounted on the charging port flex cable assembly so replacing that assembly will give you a new microphone that should cure any hardware problems associated with a bad or dead microphone.

The good news here is there aren't any components on that assembly that are paired to the motherboard so you won't lose any functionality by changing it out. Unfortunately iFixit hasn't gotten around to creating a guide for that repair for your phone, but there's a Spanish site called Nadie Me Llama Gallina that has just what you need.

Manuals / Apple iPhone 13 / Charging flex

You can buy the replacement part you need right here and support iFixit's repair efforts as well. Note that the inside of the charging port is supposed to match the exterior of the phone, so you'll want to order the correct color. Of course, any of the colors will work if you're not picky.

iPhone 13 Lightning Connector Assembly

Otherwise you can save a few bucks if you do a general internet search on that lighting port assembly; I usually recommend Amazon, eBay and AliExpress depending on your time constraints and quality vs. cost considerations.

If indeed the microphone has failed, this repair will fix you right up without causing other problems as long as you do the repair correctly. Just be careful with the display cables before you get them disconnected and be sure you have a sheet or two of precut adhesive to seal the display back up when you're done.

Good luck and let us know how it goes!