Software to edit mp3, without reducing quality?

foobar2000 has a integrated tool that rebuilds the MPEG stream. It simply analyzes the stream and removes blocks that are not MPEG blocks. This simply removes anything like ID3 tags, faulty MPEG frames and other garbage introduced by programs that don't know you to handle MPEG files.
IIRC the tags are saved and restored in the process.

If you have music encoded in MP3 with bad frames you could use this feature to remove the bad frames. This will of course not restore the faulty data. It only removes it, sometimes you won't notice this (instead of the pop / hickup the decoder produces when encountering the bad frame).

Another nice tool is mp3splt which can split MPEG streams when sync errors occur (sync error mostly occur because of corrupted MPEG frames). So you could split your file at these points. Then reconstruct the missing middle section with methods only you know
Then encode the reconstructed data and merge the three files.

Yet another tool that can rebuild the MPEG stream is mp3val (it's originally a unix tool, but I think that you can find a win32 build). It mainly does the same thing as foobar2k, sometimes better.

I always do a verification scan with foobar2k after downloading music and most of the time the encodes have no correct VBR header written to the files, which breaks gapless playback and correct display of playing time. fb2k also provides a tool to rewrite the VBR headers but sometimes refuses if there's too much additional garbage in the files.
I encountered a few files which I rebuild using the internal fb2k MPEG rebuilder but that didn't help. mp3val however did the trick.

There are even more tools:
- MP3Utility (scans your files for sync errors, etc.)
- mp3trim (editing without reencoding)
- mptrim

Jam it back in, in the dark.

When rebuilding with fb2k create a backup first. fb2k overwrites the source data while rebuilding. mp3val should create a backup of the file.

There's nowhere I can't reach.