Some of the more important issues that are found:

• broken tags / headers / audio
• duplicate tags / headers
• incorrect placement of tags / headers (ID3V2, ID3V1, LAME, Xing, …)
• low quality audio
• missing VBR header
• missing track info / cover art
• broken track info / cover art
• missing normalization data
• character encoding issues (for languages other than English)

Things to fix and change:

• Adding / fixing track info, including album cover
• Correcting files that show incorrect song duration
• Correcting files in which the player cannot seek correctly
• Converting characters for non-English names
• Adding composer name to the artist field, for players that don't handle the composer field
• Renaming files based on their fields
• Changing word case for track info

mp3check --cut-junk-start --cut-junk-end --cut-tag-end --fix-headers input.mp3

ffmpeg -i oldfile.mp3 newfile.mp3

lame input.mp3 output.mp3

lame --decode input.mp3 - | lame -V0 - output.mp3 

for f in *.mp3 ; do lame --decode "$f" - | lame -V1 - "${f%.mp3}_V1.mp3" ; done

vbrfix -always -log input.mp3 output.mp3

vbrfix -always -makevbr -lameinfo imput.mp3 output.mp3