code : pascal
uses
  MMSystem;
 
procedure TForm1.Button1Click(Sender: TObject);
var
  MixerControl: TMixerControl;
  MixerControlDetails: TMixerControlDetails;
  MixerControlDetailsSigned: TMixerControlDetailsSigned;
  Mixer: THandle;
  MixerLine: TMixerLine;
  MixerLineControls: TMixerLineControls;
  PeakMeter: DWORD;
  Rslt: DWORD;
  SourceCount: Cardinal;
  WaveOut: DWORD;
  I: Integer;
  X: Integer;
  Y: Integer;
begin
  Rslt := mixerOpen(@Mixer, 0, 0, 0, 0);
  if Rslt <> 0 then
    raise Exception.CreateFmt('Can''t open mixer (%d)', [Rslt]);
  FillChar(MixerLine, SizeOf(MixerLine), 0);
  MixerLine.cbStruct := SizeOf(MixerLine);
  MixerLine.dwComponentType := MIXERLINE_COMPONENTTYPE_DST_SPEAKERS;
  Rslt := mixerGetLineInfo(Mixer, @MixerLine,
    MIXER_GETLINEINFOF_COMPONENTTYPE);
  if Rslt <> 0 then
    raise Exception.CreateFmt('Can''t find speaker line (%d)', [Rslt]);
  SourceCount := MixerLine.cConnections;
  WaveOut := $FFFFFFFF;
  for I := 0 to SourceCount - 1 do
  begin
    MixerLine.dwSource := I;
    Rslt := mixerGetLineInfo(Mixer, @MixerLine,
      MIXER_GETLINEINFOF_SOURCE);
    if Rslt <> 0 then
      raise Exception.CreateFmt('Can''t get source line (%d)', [Rslt]);
    if MixerLine.dwComponentType = MIXERLINE_COMPONENTTYPE_SRC_WAVEOUT then
    begin
      WaveOut := MixerLine.dwLineId;
      Break;
    end;
  end;
  if WaveOut = $FFFFFFFF then
    raise Exception.Create('Can''t find wave out device');
  FillChar(MixerLineControls, SizeOf(MixerLineControls), 0);
  with MixerLineControls do
  begin
    cbStruct := SizeOf(MixerLineControls);
    dwLineId := WaveOut;
    dwControlType := MIXERCONTROL_CONTROLTYPE_PEAKMETER;
    cControls := 1;
    cbmxctrl := SizeOf(TMixerControl);
    pamxctrl := @MixerControl;
  end;
  Rslt := mixerGetLineControls(Mixer, @MixerLineControls,
    MIXER_GETLINECONTROLSF_ONEBYTYPE);
  if Rslt <> 0 then
    raise Exception.CreateFmt('Can''t find peak meter control (%d)',
      [Rslt]);
  PeakMeter := MixerControl.dwControlID;
 
  // at this point, I have the meter control ID, so I can
  // repeatedly query its value and plot the resulting data
  // on a canvas
 
  X := 0;
  FillChar(MixerControlDetails, SizeOf(MixerControlDetails), 0);
  with MixerControlDetails do
  begin
    cbStruct := SizeOf(MixerControlDetails);
    dwControlId := PeakMeter;
    cChannels := 1;
    cbDetails := SizeOf(MixerControlDetailsSigned);
    paDetails := @MixerControlDetailsSigned;
  end;
  repeat
    Sleep(10);
    Rslt := mixerGetControlDetails(Mixer, @MixerControlDetails,
      MIXER_GETCONTROLDETAILSF_VALUE);
    if Rslt <> 0 then
      raise Exception.CreateFmt('Can''t get control details (%d)',
        [Rslt]);
    Application.ProcessMessages;
    Inc(X);
    Y := 300 - Round(300 * Abs(MixerControlDetailsSigned.lValue) / 32768);
    with Canvas do
    begin
      MoveTo(X, 0);
      Pen.Color := clBtnFace;
      LineTo(X, 300);
      Pen.Color := clWindowText;
      LineTo(X, Y);
    end;
  until X > 500;
 
  // don't forget to close the mixer handle when you're done
 
  Rslt := mixerClose(Mixer);
  if Rslt <> 0 then
    raise Exception.CreateFmt('Can''t close mixer (%d)', [Rslt]);
end;
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