{ 
 Every line going into and out of the mixer has a number of "controls" 
 associated with it. Some of those controls are "meters," which give 
 you a real-time value of the sound level on the corresponding line. 
 Not all lines have meter controls, and not all sound cards provide 
 support for meters. 

 Here's some code that will retrieve a handle to the meter attached to 
 the WaveOut source of the speaker line, if there is one: 
}

 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;