mix_get_dev
device_t mix_get_dev(struct snd_mixer *m);
unsigned i = uaudio_get_child_index_by_dev(sc, mix_get_dev(m));
unsigned index = uaudio_get_child_index_by_dev(sc, mix_get_dev(m));
unsigned index = uaudio_get_child_index_by_dev(sc, mix_get_dev(m));
unsigned index = uaudio_get_child_index_by_dev(sc, mix_get_dev(m));