Why is my microphone so quiet on calls?
Usually because something between the microphone and the call is reducing the level automatically, and the most common culprits are settings nobody remembers changing.
Work through these in order:
The wrong microphone is selected. Laptops have built-in microphones, webcams have microphones, and headsets have microphones. Applications frequently default to one you are not speaking into. Check the application's audio settings, not just the operating system's — they can differ, and the app's setting wins.
Bluetooth headset profile switching. When a Bluetooth headset's microphone is used, the connection drops from high-quality audio to a low-bandwidth call profile. Audio quality noticeably degrades and the microphone can be poor. This is a protocol limitation, not a fault.
Automatic gain control fighting you. Many applications adjust input level automatically. If the operating system also has "automatically adjust microphone settings" enabled, the two can fight, producing levels that drift. Disabling the application's automatic control and setting the level manually usually stabilises it.
Input level or boost set low in the operating system's sound settings. There is frequently a separate microphone boost control that is easy to miss.
Noise suppression set too aggressively. Strong suppression can treat a quiet voice as background noise and attenuate it — which is worse for softly spoken people and in rooms with constant background sound.
Physical obstruction. The microphone port blocked by a case, a palm resting over it on a laptop, or clogged with dust. Laptop microphones are frequently tiny holes near the webcam or on the edge.
A worn or partially inserted headset jack, where the microphone contact is not seated.
Distance and direction. Microphone level falls steeply with distance — a headset boom microphone a few centimetres away will always beat a laptop microphone half a metre away, regardless of settings.
How to test properly: record yourself with any voice recorder app and listen back. That separates "my microphone is quiet" from "this particular application is quiet", which is the fork that determines everything else.