The wavemap driver wasn't passing the callback type to the

dlls/winmm/mmsystem.c

@@ -620,6 +620,8 @@ BOOL WINAPI DriverCallback(DWORD dwCallBack, UINT uFlags, HDRVR hDev,
     switch (uFlags & DCB_TYPEMASK) {
     case DCB_NULL:
 	TRACE("Null !\n");
+	if (dwCallBack)
+	    WARN("uFlags=%04X has null DCB value, but dwCallBack=%08lX is not null !\n");
 	break;
     case DCB_WINDOW:
 	TRACE("Window(%04lX) handle=%04X!\n", dwCallBack, hDev);

dlls/winmm/wavemap/wavemap.c

@@ -71,7 +71,7 @@ static void	CALLBACK WAVEMAP_DstCallback(HDRVR hDev, UINT uMsg, DWORD dwInstance
 	ERR("Unknown msg %u\n", uMsg);
     }
 
-    DriverCallback(wom->dwCallback, wom->dwFlags, hDev, uMsg, 
+    DriverCallback(wom->dwCallback, HIWORD(wom->dwFlags), hDev, uMsg, 
 		   wom->dwClientInstance, dwParam1, dwParam2);
 }