Lots of errors ...
This code works :
procedure WriteID3Tag(NewTag: ID3Struct; sFileName: string);
var
fMP3 : file;
Tag : ID3Struct;
begin { WriteID3Tag }
try
FileSetAttr(sFileName, 0);
AssignFile(fMP3, sFileName);
FileMode := 2;
Reset(fMP3, 1);
try
Seek(fMP3, FileSize(fMP3) - 128);
BlockRead(fMP3, Tag, SizeOf(Tag));

if Tag.Signature='TAG' then
{ Replace old tag }
Seek(fMP3, FileSize(fMP3) - 128)
else
{ Append tag to file because it doesn't exist.
Cannot use Append() function. It's only for text files. }
Seek(fMP3, FileSize(fMP3));

BlockWrite(fMP3, NewTag, SizeOf(NewTag))
finally

end; { try }

finally
CloseFile(fMP3)
end; { try }
end; { WriteID3Tag }

//----------------------------------------------
//error fixed, and put everything inounit:
//----------------------------------------------

un
unit ulfmp3;

interface

// The MPEG Layer 3 ID3 tag structure:
type
ID3Struct = record
Signature: array [0..2] of Char; { Should be: 'TAG' }
Title, Artist, Album: array [0..29] of Char;
Year: array [0..3] of Char;
Comment: array [0..29] of Char;
Genre: Byte;
end;

// Here's the genre (Max. 256 entries).
const
ID3Genre : array [0..125] of string = ('Blues', 'Classic Rock',
'Country', 'Dance', 'Disco', 'Funk', 'Grunge', 'Hip-Hop',
'Jazz', 'Metal', 'New Age', 'Oldies', 'Other', 'Pop', 'R&B',
'Rap', 'Reggae', 'Rock', 'Techno', 'Industrial', 'Alternative',
'Ska', 'Death Metal', 'Pranks', 'Soundtrack', 'Euro-Techno',
'Ambient', 'Trip-Hop', 'Vocal', 'Jazz+Funk', 'Fusion', 'Trance',
'Classical', 'Instrumental', 'Acid', 'House', 'Game', 'Sound Clip',
'Gospel', 'Noise', 'AlternRock', 'Bass', 'Soul', 'Punk',
'Space', 'Meditative', 'Instrumental Pop', 'Instrumental Rock',
'Ethnic', 'Gothic', 'Darkwave', 'Techno-Industrial', 'Electronic',
'Pop-Folk', 'Eurodance', 'Dream', 'Southern Rock', 'Comedy',
'Cult', 'Gangsta', 'Top 40', 'Christian Rap', 'Pop/Funk',
'Jungle', 'Native American', 'Cabaret', 'New Wave', 'Psychadelic',
'Rave', 'Showtunes', 'Trailer', 'Lo-Fi', 'Tribal', 'Acid Punk',
'Acid Jazz', 'Polka', 'Retro', 'Musical', 'Rock & Roll',
'Hard Rock', 'Folk', 'Folk-Rock', 'National Folk', 'Swing',
'Fast Fusion', 'Bebob', 'Latin', 'Revival', 'Celtic', 'Bluegrass',
'Avantgarde', 'Gothic Rock', 'Progressive Rock', 'Psychedelic Rock',
'Symphonic Rock', 'Slow Rock', 'Big Band', 'Chorus', 'Easy Listening',
'Acoustic', 'Humour', 'Speech', 'Chanson', 'Opera', 'Chamber Music',
'Sonata', 'Symphony', 'Booty Bass', 'Primus', 'Porn Groove',
'Satire', 'Slow Jam', 'Club', 'Tango', 'Samba', 'Folklore',
'Ballad', 'Power Ballad', 'Rhythmic Soul', 'Freestyle', 'Duet',
'Punk Rock', 'Drum Solo', 'Acapella', 'Euro-House', 'Dance Hall' );

function ReadID3Tag(sFileName:String):ID3Struct;
procedure WriteID3Tag(NewTag: ID3Struct; sFileName: string);

implementation


// This is the ID3 Tag read code:
function ReadID3Tag(sFileName:String):ID3Struct;
var
fMP3: file of Byte;
Tag: ID3Struct;
begin { ReadID3Tag }
try
(* sFileName - (string) The full file name with path. *)
AssignFile(fMP3, sFileName);
Reset(fMP3);
try
Seek(fMP3, FileSize(fMP3) - 128);
BlockRead(fMP3, Tag, SizeOf(Tag))
finally

end; { try }

finally
CloseFile(fMP3)
end; { try }

if tag.Signature<>'TAG' then
begin

{ Doesn't have an ID3 tag }
end { fMP3.Signature<>'TAG' }
else
begin

{ do something with the tag }
end; { not (fMP3.Signature<>'TAG') }
result:=tag;
end; { ReadID3Tag }


(* WriteID3Tag() function
**
** Copyright (c) 2000 Jacob Dybala (m3Rlin)
** Freeware.
**
** Created : January 7 2000
** Modified: January 7 2000
** Please leave this copyright notice.
*)

procedure WriteID3Tag(NewTag: ID3Struct; sFileName: string);
var
fMP3 : file;
Tag : ID3Struct;
begin { WriteID3Tag }
try
//FileSetAttr(sFileName, 0);
AssignFile(fMP3, sFileName);
FileMode := 2;
Reset(fMP3, 1);
try
Seek(fMP3, FileSize(fMP3) - 128);
BlockRead(fMP3, Tag, SizeOf(Tag));

if Tag.Signature='TAG' then
{ Replace old tag }
Seek(fMP3, FileSize(fMP3) - 128)
else
{ Append tag to file because it doesn't exist.
Cannot use Append() function. It's only for text files. }
Seek(fMP3, FileSize(fMP3));

BlockWrite(fMP3, NewTag, SizeOf(NewTag))
finally

end; { try }

finally
CloseFile(fMP3)
end; { try }
end; { WriteID3Tag }


end.