[angvelem]

Чего там отправлять?

Код:

procedure TForm1.Button2Click(Sender: TObject);
begin
  form_player.Show;
end;

ну и не забыть добавить mp3.pas в автооткрытие:
test.dpr

Код:

program test;

uses
  Forms,
  Unit1 in 'Unit1.pas' {Form1},
  mp3 in 'mp3.pas' {Form_player};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.CreateForm(TForm_player, Form_player);
  Application.Run;
end.