[marcoszorrilla]

Esta es la definición de una de las funciones que citas:

{$EXTERNALSYM mciSendString}
function mciSendString(lpstrCommand, lpstrReturnString: PChar;
  uReturnLength: UINT; hWndCallback: HWND): MCIERROR; stdcall;

la otra:

function mciSendCommand(mciId: MCIDEVICEID; uMessage: UINT;
  dwParam1, dwParam2: DWORD): MCIERROR; stdcall;

Errores que devuelve:

{ general error return values }
const
  {$EXTERNALSYM MMSYSERR_NOERROR}
  MMSYSERR_NOERROR      = 0;                  { no error }
  {$EXTERNALSYM MMSYSERR_ERROR}
  MMSYSERR_ERROR        = MMSYSERR_BASE + 1;  { unspecified error }
  {$EXTERNALSYM MMSYSERR_BADDEVICEID}
  MMSYSERR_BADDEVICEID  = MMSYSERR_BASE + 2;  { device ID out of range }
  {$EXTERNALSYM MMSYSERR_NOTENABLED}
  MMSYSERR_NOTENABLED   = MMSYSERR_BASE + 3;  { driver failed enable }
  {$EXTERNALSYM MMSYSERR_ALLOCATED}
  MMSYSERR_ALLOCATED    = MMSYSERR_BASE + 4;  { device already allocated }
  {$EXTERNALSYM MMSYSERR_INVALHANDLE}
  MMSYSERR_INVALHANDLE  = MMSYSERR_BASE + 5;  { device handle is invalid }
  {$EXTERNALSYM MMSYSERR_NODRIVER}
  MMSYSERR_NODRIVER     = MMSYSERR_BASE + 6;  { no device driver present }
  {$EXTERNALSYM MMSYSERR_NOMEM}
  MMSYSERR_NOMEM        = MMSYSERR_BASE + 7;  { memory allocation error }
  {$EXTERNALSYM MMSYSERR_NOTSUPPORTED}
  MMSYSERR_NOTSUPPORTED = MMSYSERR_BASE + 8;  { function isn't supported }
  {$EXTERNALSYM MMSYSERR_BADERRNUM}
  MMSYSERR_BADERRNUM    = MMSYSERR_BASE + 9;  { error value out of range }
  {$EXTERNALSYM MMSYSERR_INVALFLAG}
  MMSYSERR_INVALFLAG    = MMSYSERR_BASE + 10; { invalid flag passed }
  {$EXTERNALSYM MMSYSERR_INVALPARAM}
  MMSYSERR_INVALPARAM   = MMSYSERR_BASE + 11; { invalid parameter passed }
  {$EXTERNALSYM MMSYSERR_HANDLEBUSY}
  MMSYSERR_HANDLEBUSY   = MMSYSERR_BASE + 12; { handle being used
                                                simultaneously on another
                                                thread (eg callback) }
  {$EXTERNALSYM MMSYSERR_INVALIDALIAS}
  MMSYSERR_INVALIDALIAS = MMSYSERR_BASE + 13; { specified alias not found }
  {$EXTERNALSYM MMSYSERR_BADDB}
  MMSYSERR_BADDB        = MMSYSERR_BASE + 14; { bad registry database }
  {$EXTERNALSYM MMSYSERR_KEYNOTFOUND}
  MMSYSERR_KEYNOTFOUND  = MMSYSERR_BASE + 15; { registry key not found }
  {$EXTERNALSYM MMSYSERR_READERROR}
  MMSYSERR_READERROR    = MMSYSERR_BASE + 16; { registry read error }
  {$EXTERNALSYM MMSYSERR_WRITEERROR}
  MMSYSERR_WRITEERROR   = MMSYSERR_BASE + 17; { registry write error }
  {$EXTERNALSYM MMSYSERR_DELETEERROR}
  MMSYSERR_DELETEERROR  = MMSYSERR_BASE + 18; { registry delete error }
  {$EXTERNALSYM MMSYSERR_VALNOTFOUND}
  MMSYSERR_VALNOTFOUND  = MMSYSERR_BASE + 19; { registry value not found }
  {$EXTERNALSYM MMSYSERR_NODRIVERCB}
  MMSYSERR_NODRIVERCB   = MMSYSERR_BASE + 20; { driver does not call DriverCallback }
  {$EXTERNALSYM MMSYSERR_LASTERROR}
  MMSYSERR_LASTERROR    = MMSYSERR_BASE + 20; { last error in range }

Espero te sirva la información.

Un Saludo.