Hi there mercè_cid_,
If the microphones of your iPhone are no longer functioning as expected, these steps from If the microphones on your iPhone aren't working can resolve what's happening:
Clear the microphones
1. Remove any screen protectors, films, or cases from your iPhone.
2. Clear any debris from the openings of your iPhone.
If this continues after following that, force restarting your iPhone can refresh the software and resolve unexpected issues like this. This walks you through the steps: Force restart iPhone
If you are still unable to use the microphones of your iPhone, follow this section from the first article above next:
Contact Apple Support
If you still sound unclear during calls, recordings, or apps, contact Apple Support to discuss service options.
Cheers!