In a few words,
MP3 Diags is a GUI-based application that allows end-users to identify issues with their MP3 files, fix some of the issues and make other changes, like adding track information. It also lets you "look inside" an MP3 file.
Unlike some programs that are designed to deal with a single Topic (like fixing VBR headers or adding cover art),
MP3 Diags is a one stop solution that identifies more than 50 different issues in MP3 files and provides the means to fix many of them (well, not everything is fixable; you can't make a 64kbps file sound like a 256kbps one.)
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).
Some of the fixes and changes that
MP3 Diags can do:
Adding / fixing track info, including album cover; information can be retrieved from several sources: Internet, clipboard, file name, local files, and (obviously) keyboard
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.
