After I tagged a mp3 file with an external tool (Mutagen) the mp3tag started to calculate a different audio md5 for it.
Examples that differs:
https://dl.dropbox.com/u/1883230/tmp/mp3tag/original.mp3 (ID3v1+ID3v2.4)
https://dl.dropbox.com/u/1883230/tmp/mp3tag...agen-tagged.mp3 (ID3v2.4+APEv2)
Hash sums:
0CB9CBEBB7C5C2EE899E8C11B6CBF5B8 mutagen-tagged.mp3
FE7544E6F41D34A00D6B616DC418CCF9 original.mp3
Foobar2000 shows they are equal by audio data.
I decoded them to wav's with lame and they were bit-to-bit equal.
MP3Tag v2.52, Windos 7 32 bit