MPEG2 and MPEG2 Layer 3 (better known as MP3’s) are lossy file formats. This means the content is compressed to reduce file size. But this also means loss of data. To play, analyse or process the file it needs to be extracted (“de-compressed”) first. But compressing it again will result in more lost data.
To preserve the quality of the audio we’re not changing the actual data and losslessly adjust the levels of these files.