Now I can see the problems related to using different programs to edit ID3v2 entries (WM, Winamp, Helium, ID3 TagIT, etc). I lost some lyrics (USLT) and I won more than 1 ID3v1 tag per file. As I don't know the culprit, I will focus editing with Jaikoz. And only play with Winamp.
Anyway, the fact is that I have an mp3 with the following frame:
Descriptor with odd bytes (sample): FE FF 00 00 55 00 6E ... -> "Unsynchronized Lyrics" -> 00 73 00 00 (NULL terminated)
Lyrics with odd bytes (sample): FE FF 00 00 57 ...
This is the last frame.
As I understand, these are big endian strings with an extra NULL byte after the BOM. The Lyrics had a non-printable char (U+0092) that I discoved via Jaikoz and confirmed with the Hex editor. So I deleted all text lyrics and I replaced with a copy-pasted one from Firefox. After saved, the Lyrics was ok (only printable chars and new lines). But the strings remain with odd bytes (Unicode) and was not converted to Latin-1 as is in the Jaikoz preferences. The Lyrics are 100% english with 00 XX saved pattern, except the extra NULL byte.