TrackBar как регулировка громкости

//in hpp
class TForm1 : public TForm
{
__published:    // IDE-managed Components
    TTrackBar *TrackBar1;
private:    // User declarations
    void __fastcall WMNotify(
    TMessage& Msg);
public:     // User declarations
    __fastcall TForm1(TComponent* Owner);

BEGIN_MESSAGE_MAP
  MESSAGE_HANDLER(
    WM_NOTIFY, TMessage, WMNotify)
END_MESSAGE_MAP(TForm)
};
//in cpp
void __fastcall TForm1::WMNotify(TMessage& Msg)
{
  LPNMHDR pnmh = reinterpret_cast<LPNMHDR>(Msg.LParam);

  if (pnmh->code == NM_CUSTOMDRAW &&
      pnmh->hwndFrom == TrackBar1->Handle) {
      LPNMCUSTOMDRAW pDraw = reinterpret_cast<LPNMCUSTOMDRAW>(Msg.LParam);
      //ПОЕХАЛИ
      ...
  }
   TForm::Dispatch(&Msg);
}