iw_set_format
iw_set_format(sc, AUDIO_ENCODING_ULAW, 0);
iw_set_format(sc, AUDIO_ENCODING_ULAW, 1);
static u_long iw_set_format(struct iw_softc *, u_long, int);
iw_set_format(sc, p->precision, 0);
iw_set_format(sc, p->precision, 1);