Multi-channel: LFE not retained

For the multi-channel experiments (e.g. BS1534-3a, i.e. a 7.1.4 experiment) the LFE is not retained, apparently for none of the conditions (affects Direct, EVS, IVAS).

Could be tried by using e.g. the testvector from the ivas-codec repository in /scripts/testv/stv714MC48c.wav.