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Use SNDVOL32.EXE program, which is contained in windows directory. Use ShellExecute for running this program. Don't forget to add ShellAPI in uses chapter.

uses ShellAPI;
...
procedure TForm1.Button1Click(Sender: TObject);
var
  MyWin: array[0..255] of Char;
const
  Size: Integer = MAX_PATH;
begin
  GetWindowsDirectory(MyWin, Size);
  ShellExecute(
    Form1.Handle,
    'open',
    'sndvol32.exe',
    nil,
    MyWin,
    SW_SHOWNORMAL);
end;