To open a connection to the mixer device, call snd_mixer_open().
This call has arguments for selecting the card and mixer device number to open. Most sound cards have only one mixer, but there may be additional mixers in special cases.
The snd_mixer_open() call returns a mixer handle that you'll use as an argument for additional API calls applied to this device. It's a pointer to a snd_mixer_t structure, which is an opaque data type.