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Use WaveOutGetNumDevs procedure. It returns number of waveform-audio output devices present in the system.
Don't forget include MMSystem to uses chapter.

uses MMSystem;
...
procedure TForm1.Button1Click(Sender: TObject);
begin
  if WaveOutGetNumDevs<>0 then
    Label1.Caption:='Sound card is present'
  else
    Label1.Caption:='Sound card is not present';
end;

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