[dargueta]

First you need to get the handle of the sound device you're using, then pass in the handle and volume to the waveOutSetVolume function:

HWAVEOUT  hSpeaker = NULL;

//You need to fill this structure out depending on what sound you're using
WAVEFORMATEX  soundData;

//get handle to speaker
waveOutOpen(&hSpeaker,WAVE_MAPPER,&soundData,NULL,CALLBACK_NULL);

//speaker now opened for output. Set the volume.
waveOutSetVolume(hSpeaker,/*your volume here, 0xFFFF is max*/);

//do your thing.