TID3Tag = record
    ID: Array[0..3] of Char;
    Titel: Array[0..30] of Char;
    Artist: Array[0..30] of Char;
    Album: Array[0..30] of Char;
    Year: Array[0..4] of Char;
    Comment: Array[0..30] of Char;
    Genre: Byte;
    Filename: String;
    Endung: String;
  end;

  private
    { Private declarations }
    fFiles : Array of TID3Tag;

// ...

procedure TfrmMain.AddFiles(files : Array of String);
var
  len : integer;
  i : integer;
  s : String;
begin
  len := Length(fFiles);
  SetLength(fFiles, len + Length(files));
  for i := 0 to pred(length(files)) do
  begin
    fFiles[len+i] := ReadID3Tag(files[i]);
  end;
  UpdateListBox();
end;

procedure TfrmMain.UpdateListBox;
var
  i : integer;
begin
  lbList.Items.BeginUpdate;
  lbList.Clear;
  for i := 0 to pred(length(fFiles)) do
  begin
    lbList.Items.Add(trim(fFiles[i].Artist) + ' - ' + trim(fFiles[i].Titel));
  end;
  lbList.Items.EndUpdate;
end;