As far as I remember, the internal microphone of my laptop never worked. But I gave it a test again earlier today, and it seems to be now working!
However, when I record it with Audacity, the waveforms are centred around a negative number, and not around zero, as you can see here:
If I use the “Amplify” effect, the sound is naturally scaled using zero as reference and the sound is moved even more to the bottom.
Is there a way to shift a track vertically in order to centre it? (or to get my mic to work properly?)