uaudio_get_child_index_by_chan
i = uaudio_get_child_index_by_chan(ch->priv_sc, ch);
i = uaudio_get_child_index_by_chan(ch->priv_sc, ch);
unsigned i = uaudio_get_child_index_by_chan(sc, ch);
unsigned i = uaudio_get_child_index_by_chan(sc, ch);