Hello!
There are few things you may need to check:
1.step: Check your default microphone - it shouldn't be set to stereo mix or similar
2.step: Check if your headset has the mic mute switch somewhere. If you find it, set it into opposite state - you might've muted the mic by accident.
3.step: Check the volume of the default microphone - if it's very low(close to 0%), raise it up.
4.step: Check if you have plugged in the microphone into microphone connector(may not be applicable for USB headsets).
Open your panel with the TAB key multiplayer > Voice > Voice volume setting pull to the right.
Open the game. Return to the desktop.at the time sound level in the audio mixer from the speaker icon, right click on TruckersMP.
Greetings, m4gu3ll