In the documentation for snd_mixer_open, the description for the second parameter
From the source in alsa-lib/src/mixer.c:68 the "mode" argument is flagged as unused:
int snd_mixer_open(snd_mixer_t **mixerp, int mode ATTRIBUTE_UNUSED)
and the source of the function bears that out. So it looks like it's safe to use any int there (I'd personally just go with 0).