What's a "multiSound"? I assume that you want to play different audio files at the same time? Then call
PlaySound()[
^] multiple times.
You could even call it multiple times sequentially from the same thread, specifying a different sound file to play each time (see the SND_ASYNC flags, which makes the call return immediately). This way, the call will return immediately and the next sound will start playing, while the previous sound is still playing.