I'm developping a game with a synthetiser with mmsystem.h ( in C), but i don't understand everything, I just want to change the instrument but my midi message only plays PIANO everytime !!
Someone can help me ?

 for example look at my code, I use midiOutShortMsg.

union {
       unsigned long word;
       unsigned char data[4];
         } message;

   message.data[0] = 0x90;  // MIDI note-on message (requires to data bytes)
   message.data[1] = 60;    // MIDI note-on message: Key number (60 = middle C)
   message.data[2] = 100;    // MIDI note-on message: Key velocity (100 = loud)
   message.data[3] = 0;     // Unused parameter
   
    midiOutOpen(&device, midiport, 0, 0, CALLBACK_NULL);
    
    midiOutShortMsg(device, message.word);

Thanks !
(Sorry for my english)

Your question has nothing to do with XNA Game Studio or DirectX. You might try asking your question over on GameDev.net.

Check this out: http://commonmusic.sourceforge.net/doc/dict/midi-topic.html

HTH

David.