Hello xzan,
This page can help out with granting access to the microphone: Control access to hardware features on iPhone
Review or change access to the camera, microphone, and other hardware features
1. Go to Settings
> Privacy & Security.
2. Tap a hardware feature, such as Camera, Bluetooth, Local Network, or Microphone.
The list shows the apps that requested access. You can turn access on or off for any app on the list.
Note: Whenever an app uses the camera (including when the camera and microphone are used together), a green indicator appears. An orange indicator appears at the top of the screen whenever an app uses the microphone without the camera. Also, a message appears at the top of Control Center to inform you when an app has recently used either.
You may also wish to check and see if restrictions are enabled in Settings > Screen Time. If Screen Time is on, try temporarily turning it off to see if that's helped.
If other apps are able to access the microphone you may try uninstalling/reinstalling the affected app. Prior to uninstalling be sure to check with the developer for details on ensuring the data from the app is backed up, to ensure that gets preserved.
Cheers.