uses
  MPlayer, MMsystem;

type
  EMyMCIException = class(Exception);
  TWavHeader = record
  Marker1: array[0..3] of Char;
  BytesFollowing: Longint;
  Marker2: array[0..3] of Char;
  Marker3: array[0..3] of Char;
  Fixed1: Longint;
  FormatTag: Word;
  Channels: Word;
  SampleRate: Longint;
  BytesPerSecond: Longint;
  BytesPerSample: Word;
  BitsPerSample: Word;
  Marker4: array[0..3] of Char;
  DataBytes: Longint;
  end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Header: TWavHeader;
begin
  with TFileStream.Create('C:SomeFile.wav', fmOpenRead) do
  try
  ReadBuffer(Header, SizeOf(Header));
  finally
  Free;
  end;
  ShowMessage(FloatToStr((Int64(1000) * header.DataBytes div header.BytesPerSecond) / 1000));
end;