Best practise these days when considering content normalization is not to normalize to an arbitrary peak level, but to consider programme content and overall loudness when conducting normalization activities.
There are two areas of loudness that need to be considered:
Integrated loudness - Integrated loudness measurement (in LUFS) made according to ITU-...
The difference (in basic terms):
.mp3 is a compressed audio format: some of the information from the original signal is thrown away when you create an .mp3.
.wav is an uncompressed audio format. The output of the analog-to-digital conversion is stored 1:1.
when you use a converter program, it decompresses the .mp3 and creates a .wav file from the audio ...