waveOutOpen(), waveOutWrite() gives my thread undocumented message 1024 - multithreading

Question

The sound output functions are giving me unexpected messages. Is this a sign I'm doing something wrong? If so what? Otherwise is there a good source that explains what these messages might be?
waveOutOpen() gives me message 955 MM_WOM_OPEN as documented, followed by an undocumented 1024 (possibly DDM_SETFMT, DM_GETDEFID, NIN_SELECT, TBM_GETPOS, WM_PSD_PAGESETUPDLG, WM_USER, according to https://wiki.winehq.org/List_Of_Windows_Messages).
In main thread:
hAudioOut = CreateThread( 0, 0, AudioOutThreadProc, this, 0, &dwAudioOutId );
if( !hAudioOut ) {
AKS( AKSWarn, "Audio Out CreateThread() fail" );
return;
}
In the resulting audio thread:
static DWORD WINAPI AudioOutThreadProc( LPVOID lpParameter ) {
Interpreter* pinterp = (Interpreter *) lpParameter;
WAVEFORMATEX waveFormat;
waveFormat.cbSize = sizeof(waveFormat);
waveFormat.wFormatTag = WAVE_FORMAT_PCM;
waveFormat.nChannels = 1;
waveFormat.nSamplesPerSec = (int) dFreqEval;
waveFormat.wBitsPerSample = iOutputBits;
waveFormat.nBlockAlign = waveFormat.nChannels *
waveFormat.wBitsPerSample / 8;
waveFormat.nAvgBytesPerSec = waveFormat.nSamplesPerSec *
waveFormat.nBlockAlign;
MMRESULT openRes = waveOutOpen( &waveOut, WAVE_MAPPER, &waveFormat,
(DWORD_PTR) dwAudioOutId, (DWORD_PTR) this,
CALLBACK_THREAD /*| WAVE_FORMAT_DIRECT*/ );
if ( openRes != MMSYSERR_NOERROR )
Log( "waveOutOpen() = %d", openRes );
MSG msg;
int iRV;
while ( iRV = GetMessage( &msg, 0, 0, 0 ) ) {
Log( "got message %d", msg.message );
// Is the main thread asking us to stop?
if ( pinterp->bStop ) {
Log( "AudioInThreadProc(): bStop" );
return EXIT_SUCCESS;
}
// Did we get an error?
if ( iRV == -1 ) {
Log( "GetMessage() = -1: %d", GetLastError() );
abort();
}
// Did we get an expected message? (Only one expected,
// which tells us its time to write more data.)
if ( msg.message == WOM_DONE )
pinterp->Write();
// Anything else--log it.
else
Log( "got unknown message %d", msg.message );
}
Log( "AudioInThreadProc(): GetMessage = return" );
return msg.wParam;
}
waveOutWrite() isn't documented to give any messages, but is giving me message 1024 as well.