After applying some effects, like ‘Amplify’ or ‘Graphic EQ’, the central line is moved down. I use a classic Shure mic connected via USB. How to avoid it or bring it back to ‘0 line’ ? Screenshot attached.
The problem was that “silence” level of your waveform was already a bit below the centre line. It is called “DC Offset”. Amplifying has just increased (amplified) the distance.
The DC Offset needs to fixed as the first step after recording (before any editing or other processing). It can be fixed with the “Normalize” effect. See: https://manual.audacityteam.org/man/dc_offset.html